>>1300973>On systems which utilize a combination of overhead lines and third-rail for power, do units have to avoid drawing power from both sources simultaneously?Depends if the third rail and the overhead line are in the same circuit or not. In professional jargon, those are celled "sections" and there would be a short non-powered section in between because voltage difference between them would cause arching on pantograph and possibly trip breakers, as from the substation's perspective that would look like a short. I think the transition between power delivery method would be a nature place to put a section break.
Nowadays the section breaks are always designed in a way that they can be crossed on full power. I know that in old tram systems they were of such a design that the motorman had to coast while crossing section, or forementioned arching could occur.
tl;dr I don't really know for sure.
