Elsewhere in the world, 1200 V interurban / 600 V urban switchovers are still in use. One example is the Forchbahn in Zürich, which operates under 600 V when on the network of the Zürich streetcar network, and has a switchover just where it leaves the local network (in a 5% or so grade).

In the olden times, they did some series/parallel switching, and nowadays, they simply feed what they get into the 3-phase AC converters. Auxiliary drives have their own converters, which have automatic switching.

Could it be that the current drawn by an arc in a third rail DC installation is not that high? I believe remember reading that one of the recent fires at the Washington DC metro was caused by such an arc, and the current was not high enough to trip the lineside short circuit protectors…