And, as long as some current is present, they will respond properly to trains that stop, or enter and back out of a block. But they will generally introduce some drop in the voltage which actually reaches the motor. How significant this is depends on the specific technique used. The next few paragraphs will describe the principle techniques of which we are aware. In this case, the current passes through the relay coil along with the train motor and lights.
Relays have several benefits.
First, they are difficult to burn out, and, since the coil is electrically isolated from the switched contacts, properly designed relays can be used with any form of DC or even AC train control. And contacts can easily be designed to switch many amps, and thus control any type of load current. The main limitation of this technique is the issue of balancing the sensitivity of the relay with the voltage dropped across the coil. As in most areas of engineering, there are conflicting factors which must be considered. With a relay, a magnetic field of a certain strength must be generated to switch the relay.