I don't know that Lionel repairs them, but it probably can be repaired. Does it actually deliver any significant power at that low voltage? Sounds like the relay may have croaked or perhaps a bad internal connection. There is the switch and the circuit breaker relay in the path. I believe there is also a fuse in the primary winding, but I suspect if that was gone you'd get no power.
As John points out likely culprits are the fuse or the relay.
A competent service guy should be able to fix it. Plan B would be to bypass the relay and add a fast breaker external to the brick. That would cost less than a new brick. Most all the electronics pictured are there to sense over current and trip the relay. By bypassing the relay you just have an 18 volt transformer without a circuit breaker.