Question about a signal showing Restricting

ExCon90 wrote:Anybody know what the principle is for choosing one or the other?

How fast do you want the train to go?

I don't really know the answer, but I suspect it has to do with block length, since ADVANCE APPROACH requires trains exceeding Limited Speed to begin reduction to Limited Speed as soon as engine passes the ADVANCE APPROACH signal. APPROACH LIMITED only requires train to approach next signal at Limited Speed. The fact that ADVANCE APPROACH only requires one signal arm and still provide three block signalling, may also be a consideration.

Bob, remember, the most restrictive indication applies cab signals or wayside - and can't forget the letter of the rule for Advance Appraoch and Approach Limited".....

Advance Approach - "Proceed prepared to stop at the second signal. Trains exceeding Limited Speed must begin reduction to Limited Speed as soon as engine passes the Advance Approach signal."

while an Approach Limited is simply

Approach Limited - "Proceed approaching the next signal at Limited Speed."

yes, the cab signals will both allow 45mph, however, the indications are drastically different, and if the cab signals are cut out, they mean two TOTALLY different things to the engineer. an Advance Approach, the engineer has to put the brakes on as soon as he passes the Advance Approach. an Approach Limited, the engineer only has to be at 45mph by the time he hits the next signal, and then of course, the Next Governing Signal rule could apply once the next signal is clearly visible, and might not even require the engineer to slow down at all.

Approach Limited (or Medium or Slow) generally indicate a diverging/crossover movement at the next signal - these aspects are most frequently seen at distant signals. Generally they are followed by a Limited (or Medium or Slow) Clear/Approach at the next interlocking. This isn't always the case, but it is very frequently.

Advanced Approach usually indicates you are going straight and is used predominantly in shorter blocks, heavy grade and high speed territory to allow engineers to handle trains better.

The CSX RF&P Subdivision uses exclusively Approach Limited instead of Advanced Approach throughout the entire sub. An engineer told me that among the reasons listed above, it also simplifies the rules by not adding another indication. An Approach Limited gets you down to 45 by the next signal so its redundant to incorporate Advanced Approach. That's why almsot all block signals have the green bulb on the second head down, they're all capable of throwing up an App Lim even though some protect track to another block signal, not a limited speed interlocking.

Davis