What happens when you insulate the rails of a DC electric train?

[girts]

a DC electric train or tram uses a single overhead wire unlike it's AC counterparts like trolleys which use two, so if there is only a single overhead wire I assume that the return wire or let's call it ground for the electric DC train are the rails correct?

Does this mean that it is important for the rails to have good grounding ? because the train is the load and the current through it as in any DC system is proportional to voltage and resistance, since the voltage in the overhead wire is fixed the only thing that then changes is the resistance which I assume is the combined resistance of both the trains electric equipment starting from the pantograph to the lights and traction motors and then also the rail resistance? because I assume their connection electrical resistance varies from place to place, or are the rails simply grounded after intervals and the whole DC system functions like some older DC powerlines where the + power line was overhead but the -ground wire was simple used as Earth with an earthing rod at the power supply station and another one at the load side?

I assume this because the power supply stations for the DC train atleast near my city are some 30/40 km apart from one another and if the return path would be used as rails it would probably increase the total resistance and heat loses in the system by a lot since I assume rusty iron is not as good as a conductor as copper or aluminum.

Now the second part which is sort of a thought experiment also a question would be this, what would happen if one put a thin isolating plastic on the rails and the train would ride on it (slowly so that it doesn't roll away due to inertia) assuming the plastic would not break under the weight of the train, would the train be then "switched off" in other words would the train and it's electrical systems apart from backup power shut down, because such a plastic on rails would be like a blocking capacitor in the electrical schematic and we know DC cannot pass through a capacitor.
would it be dangerous because then the whole body/chassis of the train would be floating with the DC supply voltage which for the train I have is about 3kv DC?
I assume the trains electrical systems are tied to the chassis of the train as the return path since the chassis is connected to the wheels and then to rails?

 

[Windadct]

You are correct - they use the track as the return path, also for some AC systems that use single phase to the train. ( Amtrack in the Northeast of the USA uses 25hz single phase)

But they do not only use the track as the return path to the electrical substations, they will have the track sections in zones ( different electrical feeds, or branches) and have multiple bonding points between the track and the return cabling. All in addition to the tracks being electrically bonded.

In theory you could then insulate the track and turn the train off.