The Berks are, to say the least, free steaming locos in Trainz!
The thing is, keeping the boiler pressure under control on any loco (or boiler in general) requires practice, and knowing the route.
If you know you are going to be working hard, keep the coal level around 90-100% (100% is most efficient, so will produce the most steam essentially). Keep the water at 60-70%, and you may wish to open the blower just a little (press the N key - it has 6 'notches' when using the keyboard; shift+N will close the regulator).
If you know that, ahead, you are going to be working less (or coasting) let the fire burn down a bit (60-70% is a 'good' level IMO), and let the water level drop a little so you can put a bit more in to cool the boiler. Turn the blower off.
Note, putting water into the boiler will create more steam eventually, and if the fire is still hot it will convert to steam much quicker.
You can also try leaving the firebox door open to help as well, as this will let cool air into the firebox (note, not 100% sure if this actually works in Trainz; not done any specific testing of this personally).
There is a slight bug in Trainz, where the cut-off will have a largish effect on the fire temperature even when coasting. It's more just an exaggeration of the prototype (when coasting, a loco with 'drifting' valves will actually force air out creating some draft), but it will make fire control a little difficult if you aren't aware of it.
Hence, when coasting, bring the reverser back towards mid-gear (say 20% forward or reverse, depending on direction of travel). This will reduce the draft, and allow the fire to cool quicker. Some locomotives on the prototype are setup to drift in mid gear, whilst others are setup for full gear.
The thing is, to drive/fire a steamer properly you need to be familiar with both the loco and the road. Firing requires planning ahead, which is somewhat difficult when you're also driving! But, if you keep playing with the loco you will get better. I know I have my favorites that I'm starting to get the hang of. The Berks are among them, but I still get caught out on some maps!
Regards