Normally where a gas flame is used you're either plumbing with copper pipe or you need a very high temperature for Brazing, which is not technically soldering.
You cannot use a flame on PCB or rail soldering. Even a gas power portable soldering iron, while very effective (I have a well used Portasol iron), I find that the heat exchanger area (catalyst) is so hot and is located very close to the area of soldering, it can very easily melt nearby plastic.
As I've already started, for model railway general electrical soldering, the Antex 25 watt iron will be very hard to beat, both for longevity and tip interchangeability. But do get one with a Silicone cable as its far more flexible. A 45 to 60 watt digital temperature controlled iron is ideal too, but you really need to have the frequent or continuous use of one to off set its higher price.
I do not recommend anyone tries to solder wires, rails or any other model railway item with a naked flame!
Soldering wires on the BBC The Repair Shop I think the iron he is using is a possibly a Weller?