IMHO there would be nil chance of a preserved line installing third rail, whilst it might be theoretically possible, the safety management implications would kill it for a volunteer run organisation.
People have mentioned batteries, but I don't believe there is a suitable technology that exists yet. Tell me if I'm wrong.
What about putting a diesel generator into a coach body and attaching it to an EMU? If a suitable generator existed that would be a very easy solution. But there's that "if".
ISTR that SR EMUs pulled of the order of 2 000A. Let's relax that for preservation, say 1 000A. So 750V DC at 1 000A = 750kW.
What does a 750kW gen set look like? Would it fit within a Uk coach body? Is someone going to be able to find one that runs at 750V DC?
Although I guess that if a 750kW generator of a suitable size and weight existed, but wrong voltage, you could always add a rotary convertor.....
Does anyone know any more about the practicality of this approach?