Charles:
Would it be feasible to replace the 7400 in your circuit with a 74HC00. With this change it may be feasible to achieve one of the advantages that Kai talked about....that being that one size of capacitor could be chosen and then the frequency / duty cycle of oscillation could be fully controlled by resistor value changes. It is the relatively low impedance of inputs to a 7400 that make it necessary to use the 4.7K (or smaller) resistors on the gate inputs.

With the 'HC00 type part it could even be feasible to equip the PC board with a pair of 100K trimmers so that the flashing can be adjusted right at the site.

Another thing I like about the standalone nature of your design is that it can be easily setup so that as a train approaches a crossing gate a train detector or track sensing switch can be setup to feed into the input of the LED flasher. This automatically turns on the crossing lights without any special involvement by the control computer. If the detection switch is setup with two OR'ed contacts, one on each side of the crossing gate, then the lights would flash all the while there is a train detected in the crossing zone.

Michael Karas