Re:Isn't a ground a ground? Why can't they share a ground?
The rails aren't superconductors, although their resistance is low it's still a resistance, so if the vehicle draws a few hundred amps of current through them they will develop a voltage between where they are grounded at the substation and where the vehicle is touching them in accordance with ohms law. That's the reason for the rubber boots, to ensure that they ground out only at the intended point at the substation connection, so that nearby pipes and structures don't pick up current from an unintended ground path and galvanicly corrode as a result. Now imagine the skimpy little signal wires from a DC signalling circuit providing an unintended ground path for hundreds of amps. Not good...
The rails aren't superconductors, although their resistance is low it's still a resistance, so if the vehicle draws a few hundred amps of current through them they will develop a voltage between where they are grounded at the substation and where the vehicle is touching them in accordance with ohms law. That's the reason for the rubber boots, to ensure that they ground out only at the intended point at the substation connection, so that nearby pipes and structures don't pick up current from an unintended ground path and galvanicly corrode as a result. Now imagine the skimpy little signal wires from a DC signalling circuit providing an unintended ground path for hundreds of amps. Not good...
...K