Simplified Bi Di

[TheEdge]

What's the point of simplified bi-di?

All the track infrastructure is in place for full bi-di, obviously the infrastructure for signals is in place, so what possible benefit is there in signalling for simplified bi-di rather than full.

Or is it the eternal "cheaper on paper and screw reality" attitude of the railway beancounters?

 

[DarloRich]

What's the point of simplified bi-di?

All the track infrastructure is in place for full bi-di, obviously the infrastructure for signals is in place, so what possible benefit is there in signalling for simplified bi-di rather than full.

Or is it the eternal "cheaper on paper and screw reality" attitude of the railway beancounters?

probably more realistically: we only have £x authorised. Full bi di costs us £y. What CAN you deliver with the available funding.

 

[NSEFAN]

On a related note, I thought new SIMBID installations weren't allowed, given their limited usefulness due to long sections and lower linespeed when working wrong-direction? Being cheap isn't the same as good value for money.

 

[edwin_m]

Effectively the single line section constrains the frequency whatever the signalling. Having full signalling for the wrong direction would allow several wrong-direction trains to go through in quick succession but wouldn't do anything about the long time needed for the last one to clear the single line before the first one in the other direction can go through. The traffic density on Network Rail means it's virtually impossible to use it anyway, other than to support a much reduced service.

 

[Railsigns]

On a related note, I thought new SIMBID installations weren't allowed, given their limited usefulness due to long sections and lower linespeed when working wrong-direction?

New SIMBIDS installations aren't allowed but reduced capacity bi-di signalling is allowed; this is the same as SIMBIDS but with fully operational/suppressed AWS in either direction.

 

[Macwomble]

OK, I'll bite......what's SIMBIDS?

 

[snowball]

SIMplified BI-Directional Signalling.

I'll leave it to others to explain how it works.

 

[rebmcr]

OK, I'll bite......what's SIMBIDS?

simplified bi-di

.....

 

[MarkyT]

It's worth noting that SIMBIDS was a cost effective technical retrofit solution that cash strapped BR devised and used in the 1980s/90s. It allowed a basic but safe and useful signalled bi directional facility to be provided through existing relay controlled areas for pre-planned and emergency use, avoiding the difficulties and delays associated with working by pilotman. Lack of AWS with suppression for the reverse direction saved countless wires, relays and cables in that generation of equipment. There were other quirks. Auto signals in the normal direction didn't replace to red for a reverse movement, and there were no separate telephone circuits for the new reverse direction signals. The simplicity meant SIMBIDS was cheap and fast to build and commission.

It's not necessary or appropriate to avoid full bi-di control standards and train protection in new processor based signalling installations today. With their data-link networks and distributed input/output modules there are no cabling or complexity saving arguments in avoiding full bi-di controls and train protection, even if that was allowed. The question of capacity in the reverse direction is separate and largely down to how many extra signals the client is prepared to pay for, bearing in mind that wrong direction signals are sometimes more difficult to sight effectively and can require more elaborate ironwork than usual to support somewhere visible unambiguously to approaching trains.

I'm pretty certain the term SIMBIDS is no longer referred to officially anywhere in current rules and instructions, but the remaining original localities so equipped are still subject to similar restrictions on use, now written as local instructions for the particular sections. The last of these are fast being removed, as resignalling progresses on the GWML for example.

 

[LNW-GW Joint]

The old ECML SIMBIDS just shows as "Bi-directional working" on the Sectional Appendix for LNE in Northumberland, with 50mph limit in the reverse direction.
The new modular signalling systems allow bi-di, or at least it does between Shrewsbury and Nantwich, with more going in on the North Wales main line, just being installed.
Only one or two long sections of perhaps 10 miles though.

 

[Maurice3000]

Presumably the further roll out of ETCS will help bi-di workings as it requires no line-side stuff accept for balises which are already prepared for bi-di.

I can't envisage massive stretches to suddenly become bi directional but it could be very useful for passing loops, freight loops, paths for non-stopping trains through stations so they don't have to pass platforms etc. etc.

 

[MarkyT]

Presumably the further roll out of ETCS will help bi-di workings as it requires no line-side stuff accept for balises which are already prepared for bi-di.

I can't envisage massive stretches to suddenly become bi directional but it could be very useful for passing loops, freight loops, paths for non-stopping trains through stations so they don't have to pass platforms etc. etc.

Level 2 at the moment still requires conventional fixed train detection so the cost still scales with the numbers of discrete block sections required (to an extent). Future developments might result in virtual fixed blocks or even (one day but don't hold your breath) moving block. A plus point for block markers is that while they still need to be sighted for close range, long range visibility is not a concern so large complex signal structures are not required. The main difficulty at the moment is that new ETCS L2 is significatly more difficult and expensive than conventional (but latest tech) modular resignalling, especially on rural and secondary lines with longer blocks.

 

[Maurice3000]

Level 2 at the moment still requires conventional fixed train detection so the cost still scales with the numbers of discrete block sections required (to an extent).

Sure, Level 2 doesn't get rid of the fixed blocks. However, a block that maintains its length can be made bi-directional relatively easily (compared to conventional signalling) because the balises don't care about direction. The train will keep track of the order it travels over a pair of balises to establish the direction it travels in the block.

 

[MarkyT]

Sure, Level 2 doesn't get rid of the fixed blocks. However, a block that maintains its length can be made bi-directional relatively easily (compared to conventional signalling) because the balises don't care about direction. The train will keep track of the order it travels over a pair of balises to establish the direction it travels in the block.

That is true, and the fixed train detection sections limits that also define the directional block locking paths can be easily configured as natively bidirectional in the computer interlocking. It is intrinsically easier to provide for maximum capacity in the reverse direction using ETCS.

 

[Macwomble]

SIMplified BI-Directional Signalling.

I'll leave it to others to explain how it works.

Thanks.

 

[Class 170101]

SIMBIDS also means you have to be signalled in and out of SIMBIDS section.

 

[Sunset route]

SIMBIDS also means you have to be signalled in and out of SIMBIDS section.

Not in all cases, our ASC doesn’t have that requirement.

 

[snowball]

SIMBIDS is mentioned on page 74 of the November Modern Railways, in the feature on the Oxford resignalling. It says SIMBIDS "has been provided from Kennington Junction southwards to Didcot North junction, from where it continues on the up line only to Didcot station." Some full bi-di signalling is also described.

 

[MarkyT]

SIMBIDS is mentioned on page 74 of the November Modern Railways, in the feature on the Oxford resignalling. It says SIMBIDS "has been provided from Kennington Junction southwards to Didcot North junction, from where it continues on the up line only to Didcot station." Some full bi-di signalling is also described.

I doubt very much it is 'SIMBIDS', which for safety reasons cannot be be provided on new schemes. Limited capacity reverse direction signalling perhaps with not so many signals as in the normal direction, but it cannot be SIMBIDS with its complete lack of AWS for the reverse direction. The article must be incorrect.

 

[Belperpete]

The term SIMBIDS has got much misused over the years. As MarkyT notes above, the original concept was something that could be bolted onto existing relay interlockings. The simplified AWS arrangements were a critical part of the cost-saving, with no suppressors on the right-road magnets when making wrong-road moves (so the driver of a wrong-road move got a warning when passing every right-road signal), and no magnets on many of the wrong-road signals.
SIMBIDS schemes also tended to have fewer wrong-road signals, but this wasn't an inherent feature.
However, over time the term SIMBIDS has got confused. For example, the North Main Line resignalling was provided with something called SIMBIDS, even though I am fairly sure that the AWS had full suppression. I suspect that similarly the SIMBIDS that has recently been provided at Oxford is referring to a minimum of wrong-road signals, rather than any lack of proper AWS suppression.

 

[Belperpete]

That is true, and the fixed train detection sections limits that also define the directional block locking paths can be easily configured as natively bidirectional in the computer interlocking. It is intrinsically easier to provide for maximum capacity in the reverse direction using ETCS.

Yes, but what about the overlap at the end of the block? On a bidi line, you would need overlaps at both ends of each block.

 

[MarkyT]

Yes, but what about the overlap at the end of the block? On a bidi line, you would need overlaps at both ends of each block.

That is true, so you may need some addition train detection sections for bidi under ETCS along with the reverse direction block markers.
Still less trouble than with lights on sticks though.