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Re: Signal clarifications
Author: FUD
Date: 10-24-2022 - 08:56
Thanks for the clarifications regarding European practice.
Normally, station stops for US trains are short, so having a crossing signal activate isn't a large problem. In some cases, there's a timeout after which, if the train hasn't moved, the grade crossing reopens and any interconnected road traffic signals go back to normal operation; the train then has to proceed slowly until it reactivates when it's time to leave.
Behavior similar to what you describe is also seen in Sacramento. Rail signals on private r/w outside of the street-running area downtown are at stations, junctions, and dispatcher-controlled crossovers between the main tracks (there are a few manual crossovers for emergency use). Normally, they hold a train in a station, when the train indicates that it's ready to leave through wayside communications and schedule, until the grade crossings between that station and the next have activated. There are also, of course, standard track circuits for backup. The crossings are, of course, interconnected with traffic signals on the roads. Sacramento rail signals do not control light rail traffic for the normal reasons, because trains are well-separated by schedule (shortest headways are 15 minutes), and there are only a couple of single track sections left, near the ends of routes where limiting occupancy to one train works fine with the schedules. The normal operation of signals also covers the rare case of trains running close together, keeping at least one station between them. Except under emergency conditions with direct dispatcher control, there is never more than one train in a station or a train approaching an occupied station. Yes, there have been a couple of wrecks over the 30+ years they've been operating caused by violation of operating protocols (or running a red light), but they're extremely rare (as it should be; zero is the standard). I've observed similar behavior on a few other light rail systems.
One reason RT has frequently given for not running express or limited-stop trains is that they would have to completely rebuild their signal systems to support them. I'm not sure about that - it seems like there's enough space in the schedules to add some that skip a few stops, for instance. Their light rail isn't really intended for express service though - it's already skipping stops compared to buses on the parallel streets, with most stations 1/2 mile to a mile apart. With the reduced use of transit (still way down due to Covid hangover and limited reopening of downtown offices), it's even less likely that express services will be provided, whether by rail or by bus.
That, of course, works where train speeds and/or station densities are high so nobody has to wait too long for a crossing to reopen. Where trains are slow and/or stations far apart, a robust and local system - the standard track circuits - is adequate. Anything fancier is an overlay that defaults to track circuits for safety reasons.
Normally, station stops for US trains are short, so having a crossing signal activate isn't a large problem. In some cases, there's a timeout after which, if the train hasn't moved, the grade crossing reopens and any interconnected road traffic signals go back to normal operation; the train then has to proceed slowly until it reactivates when it's time to leave.
Behavior similar to what you describe is also seen in Sacramento. Rail signals on private r/w outside of the street-running area downtown are at stations, junctions, and dispatcher-controlled crossovers between the main tracks (there are a few manual crossovers for emergency use). Normally, they hold a train in a station, when the train indicates that it's ready to leave through wayside communications and schedule, until the grade crossings between that station and the next have activated. There are also, of course, standard track circuits for backup. The crossings are, of course, interconnected with traffic signals on the roads. Sacramento rail signals do not control light rail traffic for the normal reasons, because trains are well-separated by schedule (shortest headways are 15 minutes), and there are only a couple of single track sections left, near the ends of routes where limiting occupancy to one train works fine with the schedules. The normal operation of signals also covers the rare case of trains running close together, keeping at least one station between them. Except under emergency conditions with direct dispatcher control, there is never more than one train in a station or a train approaching an occupied station. Yes, there have been a couple of wrecks over the 30+ years they've been operating caused by violation of operating protocols (or running a red light), but they're extremely rare (as it should be; zero is the standard). I've observed similar behavior on a few other light rail systems.
One reason RT has frequently given for not running express or limited-stop trains is that they would have to completely rebuild their signal systems to support them. I'm not sure about that - it seems like there's enough space in the schedules to add some that skip a few stops, for instance. Their light rail isn't really intended for express service though - it's already skipping stops compared to buses on the parallel streets, with most stations 1/2 mile to a mile apart. With the reduced use of transit (still way down due to Covid hangover and limited reopening of downtown offices), it's even less likely that express services will be provided, whether by rail or by bus.
That, of course, works where train speeds and/or station densities are high so nobody has to wait too long for a crossing to reopen. Where trains are slow and/or stations far apart, a robust and local system - the standard track circuits - is adequate. Anything fancier is an overlay that defaults to track circuits for safety reasons.
