For steam, I would only say that if you are dealing with brass, you need to be very careful. These parts can be very fragile. In some cases, there are little screws that allow dismantling - I would do that if I were to apply force on a wheelset. I have two engines with broken trailing wheel trucks (from previous owners), and these are not easy to fix or replace.
Simon
There is a tool made for this.
What tool? I am not aware of any tool that allows you to re-gauge wheels that are still on their trucks. Apart from a gauge, of course.
Lastspikemike Diesels have half axles press fitted into the central gear. You can change the gauge by twisting the wheels relative to each other and either pressing in or pulling outwards, in theory. I've not tried this. I'm pretty sure it's hard to move the gauge. For some diesels it might be easier to buy already gauged wheelsets with gears installed. Walthers sells some types and Athearn sells gears, not sure about wheelsets.
Diesels have half axles press fitted into the central gear. You can change the gauge by twisting the wheels relative to each other and either pressing in or pulling outwards, in theory. I've not tried this. I'm pretty sure it's hard to move the gauge.
For some diesels it might be easier to buy already gauged wheelsets with gears installed. Walthers sells some types and Athearn sells gears, not sure about wheelsets.
A German Co. Fohrmann Werk makes a fixture for such and NW Short Line makes something similar.
DMK
The OP appears to have stated that he wants to do this task while the wheels are in place on the locomotive, or at least still mounted in their truck frames. NOTE HIS USE OF THE PHRASE: "...while in the trucks."
I printed this in ALL CAPS because it appears (some) people don't want to read that on which they are expressing opinions.
The NWSL tool cannot do this. The Fohrmann Werk tool cannot do this.
A POSSIBLE exception might be steam engine lead and trailing wheels that have inboard bearing trucks.
I have my doubts that you can twist diesel half-axles while the wheels are in place in the trucks.
Ed
This tool perhaps?
https://www.fohrmann.com/en/wheel-spacer.html
But this cannot be used for wheels on trucks. The NWSL gear puller, a great tool by the way, is similar.
Thanks