what happens if a tram breaks down

[edwin_m]

Articulated lorry tractors move 40 (44?) tonnes gross, so why not a rescue tractor? I thought rubber tyres on tarmac had better adhesion than steel wheels on steel rails.

That only works when the entire rescue journey is on street, which is unlikely on most of Metrolink particularly as the depots are on sections of ballasted track.

And waiting for a rescue tractor doesn't address the inconvenience to passengers of abruptly taking out of service a tram that was going in the opposite direction.

A tram failure is going to stop the service for a considerable period of time however it is dealt with. This is usually minimised by having the tram behind couple up and push - two trams out of service but by the time it's achieved there will be at least one more close behind. It would be unusual to borrow a tram going the other way, as most failures won't be near a convenient crossover and it's probably better to use a following vehicle that can't be doing anything else due to being blocked by the failure.

Even if you could control the hydraulics from a towing vehicle, it only kicks in at around 5mph and below, everything else Is regenerative, so you still have 40 tons pushing against the recovery vehicle.

Not really relevant to this discussion, but for the record the Bombardier Croydon trams, and I assume also the very similar Metrolink ones, are able to stop from maximum speed using friction brakes alone. I've seen it happen when testing them when new - though the brakes got pretty hot.

The parking brake being audible every time a tram stops or starts, so all Metrolink passengers will be aware of it, but other than that, do they purely use rheo/regen for deceleration with no friction or air braking? I have been on a tram which was in the unfortunate enough situation to hit someone, so I have experienced an emergency brake application (and the driver made an announcement that we would not be able to move again until he had reported having made an emergency brake application) and it is really very sharp, not at all like train brakes requiring miles to stop.

Emergency (hazard) braking uses a third system, electromagnetic track brakes that rub on the rail itself and therefore do not depend on wheel-rail adhesion. This gives about 25%g deceleration, compared to a train which achieves 12%g maximum in emergency. Service brakes on trams are also more powerful and more controllable than train brakes, for safe operation on the street.

 

[XAM2175]

I was really hoping that the answer would be that a Unimog or something similar with a tram coupling would streak through the traffic, back down on the failure and tow it to the depot, minimising the delays to all the other trams in the queue.

If it's any consolation, you do see that in Melbourne. The recovery trucks carry a team of fitters and a selection of commonly-needed spare parts, but can also tow or shove a tram if needed. It's not so much of a problem there because all but a few small parts of the network are street-running.

Getting a push from a behind obviously also remains an option, but since they're not fitted with couplers it requires the fitters to attend anyway.

(by Marcus Wong at https://railgallery.wongm.com/melbourne-trams-road-fleet/E122_4961.jpg.html)

(by Marcus Wong at https://railgallery.wongm.com/melbourne-trams-cbd-and-docklands/F117_3340.jpg.html)

 

[Mothball]

on the railway a continuous brake is one that applies the brakes on both portions of the train in motion if it parts. A parking brake is a bit different.
I have never heard of a continuous hydraulic brake, but if it is just electrically powered and controlled then why not have a power supply from the assisting vehicle? Sounds like a specification or design failure to me.

then can an assisting vehicle (Tram or unimog) really not supply enough power to the failed vehicle so that the brakes will work?

During normal operations, for all intents and purposes it is a continuous brake, just controlled by electrical signals via the couplings rather than air piping. If the vehicle is electrically isolated then you can't send it electrical signals, therefore can't control the brakes....

Not really relevant to this discussion, but for the record the Bombardier Croydon trams, and I assume also the very similar Metrolink ones, are able to stop from maximum speed using friction brakes alone. I've seen it happen when testing them when new - though the brakes got pretty hot.

On the M5000s the friction brakes on the motor bogies are essentially on or off, the regen does the brunt of the work. I'd have to check but I'm fairly certain the friction brakes apply fully when making a hazard brake regardless of speed rather than just when coming to a stop as normal.
The trailer bogie is a different set up to account for the lack of regen, with extra callipers that provide variable levels of braking regardless of speed.

 

[507 001]

do they purely use rheo/regen for deceleration with no friction or air braking? I have been on a tram which was in the unfortunate enough situation to hit someone, so I have experienced an emergency brake application (and the driver made an announcement that we would not be able to move again until he had reported having made an emergency brake application) and it is really very sharp, not at all like train brakes requiring miles to stop.

Basically yes, with a couple of exceptions in more unusual circumstances. The lack of blending is why you get the horrible lurch when you stop on a gradient.

Hazard braking throws everything in thought, regen, disks and track brakes.

It appears that the M5000s do not require a continuous air brake of the Westinghouse type as heavy rail vehicles do.

They do in the same way that anything fitted with westcode EP brakes does, via the train wires.

During normal operations, for all intents and purposes it is a continuous brake, just controlled by electrical signals via the couplings rather than air piping

See above. It’s my understanding that an emergency brake application on westcode EP braked stock would be exactly the same as on an M5000, by interrupting the train wire. The air connection on westcode fitted stock is the Main Res, not the train brake pipe.

I'd have to check but I'm fairly certain the friction brakes apply fully when making a hazard brake regardless of speed rather than just when coming to a stop as normal.

They do.

with extra callipers that provide variable levels of braking regardless of speed

We’re told in training that the disk braking on the trailers works the same way as on the motors and doesn’t engage till you’re at a stand?

 

[AndrewE]

That [rubber tyres on tarmac] only works when the entire rescue journey is on street, which is unlikely on most of Metrolink particularly as the depots are on sections of ballasted track.

I would have expected drop-down rail wheels like lots of road/rail on-track machines

A tram failure is going to stop the service for a considerable period of time however it is dealt with. This is usually minimised by having the tram behind couple up and push - two trams out of service but by the time it's achieved there will be at least one more close behind. It would be unusual to borrow a tram going the other way, as most failures won't be near a convenient crossover and it's probably better to use a following vehicle that can't be doing anything else due to being blocked by the failure.

I think we were told that they have to take one from the other direction too, and run it over a crossover ("one every 3 stops?") then set back against the traffic to haul the cripple as well as having assistance in the rear.

Emergency (hazard) braking uses a third system, electromagnetic track brakes that rub on the rail itself and therefore do not depend on wheel-rail adhesion. This gives about 25%g deceleration, compared to a train which achieves 12%g maximum in emergency. Service brakes on trams are also more powerful and more controllable than train brakes, for safe operation on the street.

I thought the track brakes depended on shorting the OLE through the electromagnet brakes, so I hadn't imagined a failed tram having the pantograph up anyway.
I would have expected the couplings to have a DC line at least, so that (specifically in these circumstances) there was power in a crippled unit to make the rheostatic brakes work. Not regen, obviously as the cripple could be minus its pan!

 

[507 001]

thought the track brakes depended on shorting the OLE through the electromagnet brakes, so I hadn't imagined a failed tram having the pantograph up anyway.
I would have expected the couplings to have a DC line at least, so that (specifically in these circumstances) there was power in a crippled unit to make the rheostatic brakes work. Not regen, obviously as the cripple could be minus its pan!

The track brakes are just big electromagnets that drip down onto the track. They run off the vehicles batteries so don’t rely on the traction supply at all.

You really want to put a 750v DC Bus line through the couplers?! That’s not even done on heavy rail, never mind light rail where they’re much more accessible to pedestrians…

 

[edwin_m]

I would have expected the couplings to have a DC line at least, so that (specifically in these circumstances) there was power in a crippled unit to make the rheostatic brakes work. Not regen, obviously as the cripple could be minus its pan!

For the rheostatic brakes to work, the traction system also has to be working, as it uses the same motors and most of the same electronics. So any failure that prevents the tram moving probably also renders the rheostatic brakes useless too.

 

[Mothball]

We’re told in training that the disk braking on the trailers works the same way as on the motors and doesn’t engage till you’re at a stand?

The trailers are a different setup, they have a larger primary brake unit than the motors and an accumulator to store the extra energy required. That's connected to the silver pipe that runs around the outside of one side of the bogie.
The Auxiliary unit for the spring applied release is the same as the motors.
Couldn't say if they make much of a difference against the regen, the pads on the trailers certainly wear down quicker than the motors.

 

[507 001]

For the rheostatic brakes to work, the traction system also has to be working, as it uses the same motors and most of the same electronics. So any failure that prevents the tram moving probably also renders the rheostatic brakes useless too.

This is the point isn’t it. It doesn’t matter if Rheo/Regen is working if you can’t even get the parking brake to release, never mind take traction!

The trailers are a different setup, they have a larger primary brake unit than the motors and an accumulator to store the extra energy required. That's connected to the silver pipe that runs around the outside of one side of the bogie.
The Auxiliary unit for the spring applied release is the same as the motors.
Couldn't say if they make much of a difference against the regen, the pads on the trailers certainly wear down quicker than the motors.

Interesting! Thanks for the info!

 

[Mothball]

these are all very intresting comments thankyou

what kinds of trailers do they use ? thanks

 

[edwin_m]

what kinds of trailers do they use ? thanks

Trailer just refers to the middle section, whose wheels do not have motors and are therefore, in railway parlance, "trailing" even if they are in the middle.

 

[Mothball]

tjankyou very much edwin

 

[Townsend Hook]

I'm sure I've seen a Unimog (on tyres) towing a tram in Blackpool
but maybe it's safer there: no real hills

There may well have been a driver at the controls of the tram, able to stop it on the handbrake in an emergency? I'm assuming it was one of the traditional trams and not one of the modern Flexity light rail vehicles?

 

[Thirteen]

Come to think of it, I've never seen any of the Trams in London break out or get towed away.

 

[Mothball]

Come to think of it, I've never seen any of the Trams in London break out or get towed away.

i have never been to london but i saw a picture of one of the london trams being towed but i think it was being tested so i supose it is a bit of a different proses

 

[philthetube]

There may well have been a driver at the controls of the tram, able to stop it on the handbrake in an emergency? I'm assuming it was one of the traditional trams and not one of the modern Flexity light rail vehicles?

Tyres would be the preferred option on non ballasted track, much better grip with rubber than metal, useful if towing something much heavier than yourself.

 

[D365]

If a Tram breaks down on the Sheffield Supertram another Tram comes along & tows it to the depot.

Correct. I have seen a Siemens tram rescuing a Citylink.

 

[Taunton]

San Francisco cablecars have a couple of push trucks, normally visible at the depot, which come out to various events. They used to be old and battered Mack trucks, but seems to have moved on to something more up to date in this Youtube video. Surprising that passengers were left on board while the pushing took place. Here (click):