Manchester: near miss between two trams, 17 May 2019

Adlington

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Another job for the RAIB:
At around 17:19 hrs on 17 May 2019, a Manchester Metrolink tram travelling inbound from Manchester Airport towards Victoria passed through the centre platform of Deansgate-Castlefields tramstop. It was required to stop at the platform, but did not do so.

It then passed a signal displaying a stop aspect at the far end of the platform, while travelling at around 9 mph (14km/h), and entered the section of line that leads to St. Peter’s Square tramstop.

Passing this signal placed the tram in a conflicting movement with another tram, which was departing from the other inbound platform having been allowed to do so by the signalling system. The driver of the second tram realised what was happening and bought their tram to a stop before a collision occurred. The tram which passed the signal at stop continued on to St. Peter’s Square tram stop.

There were no injuries or other adverse consequences resulting from this incident. However, a collision between the two trams was only narrowly avoided.
SPAD on a tramline....
 
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507 001

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Technically, it wasn’t a SPAD as our signals do not display a Danger aspect, rather a Stop aspect.

Therefore it is a SPAS.
 

Jozhua

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How did it manage to pass through deansgate and a stop signal and keep going towards st peters square without the driver realising? Sounds like human error to me, or perhaps if it was late they were just speeding through on purpose?

From what I can gather the Metrolink trams each broadcast their route plan to local recievers which control the signalling and points for each section to make sure the trams get to the right place. I don't know what sort of effect this would have on the incident, but if anyone knows more about the Metrolink signalling system it would perhaps help us understand the incident a bit better!
 

edwin_m

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How did it manage to pass through deansgate and a stop signal and keep going towards st peters square without the driver realising? Sounds like human error to me, or perhaps if it was late they were just speeding through on purpose?

From what I can gather the Metrolink trams each broadcast their route plan to local recievers which control the signalling and points for each section to make sure the trams get to the right place. I don't know what sort of effect this would have on the incident, but if anyone knows more about the Metrolink signalling system it would perhaps help us understand the incident a bit better!
It is roughly as you describe but the important point is that there is no form of enforcement of stop signals like TPWS on the main line (except for the short section at Altrincham that still has railway-like signalling unless it's changed recently). The signals are similar in principle to traffic lights - two of them leading to the same piece of track shouldn't clear at the same time but even if the signal is showing proceed it's still up to the driver to stop if the track is obstructed. However there was probably a risk here that if one tram was slightly earlier or later relative to the other one, neither driver would have seen the other tram in time to prevent a collision.
 

Starmill

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Given the tramway is operated on line of sight, it is unclear how an automatic system to stop the tram in an emergency could ever work. There are various pieces of technology used to make the tram management system more efficient while keeping it sufficiently safe, for example variable message signs. I do not know what state the more advanced features of that are currently at. Fundamentally though, it will always be up to the driver to use their eyes to ensure they do not drive into an obstruction, another road user, or another tram on the line ahead of them.
 

edwin_m

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Given the tramway is operated on line of sight, it is unclear how an automatic system to stop the tram in an emergency could ever work. There are various pieces of technology used to make the tram management system more efficient while keeping it sufficiently safe, for example variable message signs. I do not know what state the more advanced features of that are currently at. Fundamentally though, it will always be up to the driver to use their eyes to ensure they do not drive into an obstruction, another road user, or another tram on the line ahead of them.
You could enforce compliance with signals and significant reductions in speed, and the latter may well become normal practice as a result of the Croydon accident. And if you can have an autonomous car then you can have an autonomous tram - you just have to programme it to know it can't steer! But this is some way off for either cars or trams. Radar-based adaptive cruise control as fitted to many cars probably wouldn't help, as accidents tend to involve something unexpectedly entering the swept path from one side rather than colliding with a vehicle in front following the same course.
 

Starmill

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As I understand it, part of the switch to line of sight operation has already reduced the maximum permissible speed on a number of sections of the route. This still theoretically provides for greater capacity, and offers a much cheaper installation than (say) a moving block system would have.
 

edwin_m

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It could be worse: Wales is lagging even further behind: we're not only driverless, but tramless too!

(Unless you count the funicular railway on the Great Orme.)
The Great Orme would fall under the legal definition of a tramway that I posted recently in another thread, and I think it describes itself as such. It is unfenced and part of it is in a public road, so if there was an obstruction the driver would have to stop the vehicle - though I'm not sure exactly how, considering it's powered by a fixed winding engine.

You'll have half a mile or so more tramway when the Cardiff Bay branch is converted and extended.
 

sprunt

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It is roughly as you describe but the important point is that there is no form of enforcement of stop signals like TPWS on the main line
Has this always been the case? I am sure that I remember an occasion, probably about 20 years ago, when I was on a Bury tram approaching Victoria from the north when the driver announced that due to issues with the signalling, he would need to go through a signal against the tram that would cause automatic braking so we should all hold on tight. He proceeded slowly and as he warned, the tram stopped abruptly.
 

Bletchleyite

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The Great Orme would fall under the legal definition of a tramway that I posted recently in another thread, and I think it describes itself as such. It is unfenced and part of it is in a public road, so if there was an obstruction the driver would have to stop the vehicle - though I'm not sure exactly how, considering it's powered by a fixed winding engine.
There's some kind of radio contact with the winding room. Used to be an overhead wire (which is why people get confused thinking it's a conventional tram) but now radio. I'd imagine that includes some kind of emergency stop feature.
 

Bletchleyite

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Has this always been the case?
I don't think so. The ex-railway sections of Metrolink used to use block signalling, but they have now been converted to "drive on sight", though I'm surprised that there isn't an automatic stop feature for conflicting movements (as distinct from a simple rear ending) as in such situations the other tram can't be assumed to see the problem and stop, though fortunately in this case the driver did.
 

edwin_m

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I don't think so. The ex-railway sections of Metrolink used to use block signalling, but they have now been converted to "drive on sight", though I'm surprised that there isn't an automatic stop feature for conflicting movements (as distinct from a simple rear ending) as in such situations the other tram can't be assumed to see the problem and stop, though fortunately in this case the driver did.
The block signalling did include a train stop feature. Some of the 3xxx trams were fitted but not all, hence the restriction on some of the fleet using the Altrincham line and the Bury line although I believe that has now been converted to line of sight.
 

edwin_m

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There's some kind of radio contact with the winding room. Used to be an overhead wire (which is why people get confused thinking it's a conventional tram) but now radio. I'd imagine that includes some kind of emergency stop feature.
I'd hope there's something more reliable than that. Perhaps some kind of load sensor on the winding machine so it stops if the load is unsually high or low due to one of the trams having brakes applied.
 

duffield

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I'd hope there's something more reliable than that. Perhaps some kind of load sensor on the winding machine so it stops if the load is unsually high or low due to one of the trams having brakes applied.
I'd guess maybe you could have an emergency brake which would switch from clamping the moving cable to clamping a fixed cable? (Not saying this is how it actually works!)
 

Greybeard33

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The block signalling did include a train stop feature. Some of the 3xxx trams were fitted but not all, hence the restriction on some of the fleet using the Altrincham line and the Bury line although I believe that has now been converted to line of sight.
Conversion of the Bury and Altrincham lines to line of sight has been proceeding very slowly, in stages.

The Bury line is still block signalled, with Automatic Tram Stop (ATS), between Whitefield and Bury. It is intended to convert this final section once Thales Tram Management System (TMS) signalling specialists become available from higher priority projects. There is a barriered level crossing at Hagside that is a complicating factor.

The Altrincham line is still block signalled, with ATS, from Brooklands to Altrincham outbound and from Altrincham to Timperley inbound. Network Rail (Deansgate Junction box) controls the signals on the Deansgate Junction to Altrincham section, which includes two barriered level crossings. The formation through Navigation Road station and the crossings consists of two parallel single tracks for Metrolink and heavy rail. There is no published schedule for conversion of this Network Rail section to line of sight; I suspect it will be postponed until the eventual abolition of the DJ box and transfer to the Ashburys ROC. Meanwhile it is planned to extend line of sight to the Metrolink/Network Rail interface just north of Deansgate Junction, but the required TMS implementation is low priority, like Whitefield to Bury.

As you say, only half of the M5000 tram fleet (3001-3060) is equipped with ATS. The remaining vehicles are not allowed on the Bury or Altrincham lines, which complicates rostering.
 

edwin_m

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I'd guess maybe you could have an emergency brake which would switch from clamping the moving cable to clamping a fixed cable? (Not saying this is how it actually works!)
Apparently (http://www.greatormetramway.co.uk/en/how-it-works):
The tram staff are in constant and direct contact with each other. If an issue were to arise, the winchmen and attendants are able to signal that the tram needs to stop. The brakes are applied, bringing the tramcar to a full stop. In the event that the cable hauling the tramcar were to malfunction, the tramcars are also equipped with automatic emergency breaks. These brakes stop a tramcar within a metre.
Mentioned elsewhere on that page that it is an induction loop, so probably something a bit more than just a walkie talkie!
 

507 001

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Driverless trams are used abroad without safety compromise, as usual England lags behind.
Where?! News to me!

I don't think so. The ex-railway sections of Metrolink used to use block signalling, but they have now been converted to "drive on sight", though I'm surprised that there isn't an automatic stop feature for conflicting movements (as distinct from a simple rear ending) as in such situations the other tram can't be assumed to see the problem and stop, though fortunately in this case the driver did.
As Greybeard has said, not all of the Bury and Alti lines have been resignalled. We’re still on block signalling In a couple of relatively small areas.

It’s line of sight driving. Therefore speeds have been reduced to such that you can stop in such a situation.

The block signalling did include a train stop feature. Some of the 3xxx trams were fitted but not all, hence the restriction on some of the fleet using the Altrincham line and the Bury line although I believe that has now been converted to line of sight.
Not all of it. Still Block north of Whitefield (thankfully).


It’s worth noting that most converging junctions and single line sections have SPAS indicator lights. These are essentially blue flashing beacons that activate if a signal is passed at stop. Unfortunately they are not fitted in this area as speeds are deemed to be low enough (8mph coming off the Inbound, 10 off the centre road).
 

edwin_m

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It’s worth noting that most converging junctions and single line sections have SPAS indicator lights. These are essentially blue flashing beacons that activate if a signal is passed at stop. Unfortunately they are not fitted in this area as speeds are deemed to be low enough (8mph coming off the Inbound, 10 off the centre road).
Good point, and I should add that where fitted they are visible to drivers of other trams as well as the one triggering them, and everyone seeing them lit must stop immediately. So they should also stop other trams that might be on a collision course.
 

Jozhua

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Just wondering, why was the other signalling system removed in favour of line of sight when clearly it results in a reduction in speed and perhaps safety?

I understand the need for increases in capacity at certain times, but couldn't the system switch between line of sight and standard signalling procedures at specific times or just ensure all trams are doubled up when extra capacity is needed?
 

MetroCar4058

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Just wondering, why was the other signalling system removed in favour of line of sight when clearly it results in a reduction in speed and perhaps safety?

I understand the need for increases in capacity at certain times, but couldn't the system switch between line of sight and standard signalling procedures at specific times or just ensure all trams are doubled up when extra capacity is needed?
The numbers MetroLink needed to put through some of these stretches were much greater than the block signalling could cope with.
 

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