I've started using the Silver solder paste which makes the job a lot easier
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As mentioned it's perfectly feasible to do with soft solder but if then soldering on wires and wing rails you just need to be quick and confident.
I do a lot of my small details with silver-solder, lamp irons brackets etc. because I then soft-solder them to the loco without any fear of the joint failing at soft solder temperatures, the main advantage of a silver-soldered joint. Also as mentioned the joint will be much harder than a soft soldered joint so less prone to wear but only really a consideration if expecting intensive usage. The disadvantage is that silver solder will not fill gaps like soft solder does, so the joints do need to be a reasonably good fit to get the solder to flow, they don't need to be high precision but you just can't get away with agricultural engineering like you can with soft solder.
As for the jig - you don't need anything fancy nor does it need to stand up to the high heat. When I did my crossing V's (7mm) with silver solder I simply cut a small piece of nickel-silver sheet in a trapezium shape - the non-parallel sides cut to the required angle of the crossing V. This can then sit an inch or so away from the joint and the rails are fastened to the "jig" using binding wire.
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If you are silver soldering then the binding wire is really useful to get - it allows you to hold the components together whilst soldering.