Riding up front may be curtailed whenever special circumstances occur that require the drivers full attention. I'm sure that's all it is with the lightning.
As far as the safety issue is concerned, being in a monorail (or an automobile) is one of the safest places you can be during a lightning storm. I saw a presentation on this at the Boston Museum of Science many years ago, where they specifically talked about being in a car.
It has nothing to do with rubber tires, which would have to be "a mile thick" to provide enough insulation to stop a lightning bolt. It's because electrical current flows along the outside of a metal surface in this situation. To demonstrate this effect, one of the Museum staff sat in a metal mesh cage (somewhat like an electrical cage in a theatre) suspended from the ceiling while visible electrical bolts of roughly a million volts from a large Van de Graff generator struck the cage. The staff member then placed her hands flat against the inside of the metal mesh while the bolts were hitting the cage!! And of course she was completely unharmed, because the electricity flowed along the outside of the mesh, no more than an inch from her hand.