Closer running: is it feasible?

[whole diary]

[grahame]

From RailEngineering

Interesting article.  I'll quote just the introduction and let you read into the detail

Railways around the world are facing demands to transport more passengers and freight, but constructing new tracks is both expensive and unpopular. This leads to an ambition to run more trains on existing tracks, and challenges to the fundamental principles of present day railway operations are being proposed to achieve this goal.

One such principle is that trains following one another on the same track must be separated by a sufficient margin to ensure every train is capable of braking to a stand before reaching the last known position of the train in front. With braking rates limited by the adhesion between steel wheel and steel rail, the required separation increases dramatically with speed ^ on a high-speed line trains must run several kilometres apart. The migration from fixed block to moving block signalling (such as CBTC^▸ (Communications-based train control) or ERTMS^▸ (European Rail Traffic Management System.)/ETCS^▸ (European Train Control System) level 3) is driven by an ambition to maximise capacity within this constraint, but demand for ever increasing capacity means that the principle itself is now under challenge.

The argument for an alternative approach starts with the assertion that it is unrealistic to assume the train in front will stop instantly. Provided the two trains have similar braking rates, the trains can run much closer together.

[broadgage]

Moving block signalling does of course permit of running trains much closer together and can give a useful increase in capacity.
If correctly applied, no reduction in safety standards is implied, in front of the second train there will always be sufficient distance in which to stop. This distance can be surprisingly short at low speeds.

However what is proposed above DOES seem to imply a less safe method of working. It is proposed that the distance between two trains moving at significant speed could be reduced on the grounds that the front one wont suddenly stop.
Under normal conditions, the front train simply cant stop dead without warning, there is a definite and known limit to the deceleration that the brakes can apply. Any anyway basic physics limits it due to the available friction.

Unfortunately this seems to take no account of the consequences if the front train DOES stop almost instantly perhaps due to derailment and striking a bridge abutment. The consequences are bad enough with but a single train, but consider at least one and perhaps several following trains running into the wreckage.
And if the wreckage is foul of the other line, SEVERAL trains running in the other direction could also be wrecked.

It would reduce safety standards to those prevailing on motorways, which are among the safest roads, but still much less safe than railways.
Very few drivers on a motorway can reliably stop in the distance seen to be clear, especially in poor weather. This normally works OK with vehicles following each other closer than is "safe" because the braking rate of the front vehicle is at least roughly known and the following driver knows this.
Sometimes however something goes wrong and a vehicle DOES stop much more quickly than expected, perhaps as a result of a tyre bursting, or a side swipe collision. A multi vehicle pile up then results sometimes with multiple lives lost.

[stuving]

Unfortunately this seems to take no account of the consequences if the front train DOES stop almost instantly perhaps due to derailment and striking a bridge abutment. The consequences are bad enough with but a single train, but consider at least one and perhaps several following trains running into the wreckage.
And if the wreckage is foul of the other line, SEVERAL trains running in the other direction could also be wrecked.

Yes it does, though not in much depth. It also points out that no protection is currently implemented against a train on another line that could strike a derailed train.

What does seem to be the strongest counterargument, at least in that article, is the need to wait until points are detected in position before allowing a train within braking distance. That, combined with the need to split train flows over more platforms if they are closer, leads to returns that diminish before they appear.

[John R]

One only has to think of the Potters Bar crash where the train stopped more or less instantly to realise that the risk of a multiple collision is increased significantly if the assumption is made that the train in front will always stop in a controlled manner.  As is noted, you cannot sensibly lower the risk in that situation of a train happening to pass in the opposite direction,  but I would have thought it undesirable to introduce a change that increases a risk that is completely manageable.

[Bmblbzzz]

The obvious response is that this is how almost all road traffic operates, against the best advice of the Highway Code and despite* measures such as distancing chevrons painted on carriageways. Buses and coaches drive like this too, so if it is safe enough for them, why not for railways? I'm not sure of the rules or practices concerning landings at busy airports, but watching them, a following plane certainly looks committed to landing before the preceding one has cleared the runway. That could be misleading though.

Anyway, these concerns are addressed in the article. As is its reliance on automatic driving and perhaps most importantly, that it probably would not raise capacity in practice.

*It could be that these measures in effect legitimise the behaviour by showing an advised spacing which is itself usually in breach of the advice.

[Chris from Nailsea]

Forty seconds, according to West Coast Railways.



Sorry: did I say that out loud??   

[John R]

Digressing slightly given the mention of WCR, a few days ago my service from Swindon to Bristol ran wrong line from North Somerset Jn to Bristol East Jn.  Rather puzzled at a very unusual move, I noticed a rake of WCR stock and a Class 47 in the Kingsland Rd sidings with staff walking around so obviously very recently moved there.  I did give a wry smile at the thought of the GWR^▸ (Great Western Railway) service being routed away from the adjacent line to the WC^▸ (Wiltshire Council (Unitary Authority)) train, such is their reputation these days.