Hi Xenithar
Steam locomotives definitely work, albeit with a few shortcomings, in TRS19.
The main shortcomings are a lack of a damper control unfortunately, boiler pressure remaining stagnant when the boiler goes cold when not using steam, and some potential limitations to adjusting steam usage (which affects coal/water usage).
However, there are unfortunately many locomotives do not have great enginespec assets, and as such they may either perform unrealistically or are difficult to drive realistically. The biggest one is that locomotives are poorly configured for steam generation rates and safety valves. There is a LOT of experimentation needed to get these right, but it can be done to a reasonable level in most cases.
The biggest two issues are that locos either generate steam too easily (or do not use enough steam, same thing for the most part but different parts of the espec causing it), or their safety valves are very poorly setup (either with too high or too slow a rate).
Some locos are setup to release a massive amount of steam from the safety valves very quickly, which causes the boiler pressure to rapidly increase and drop. This results in a constant 'pst pst pst pst pst' from the safety valves when driving; unless you manage to slow the steam generation a bit.
Others release pressure too slowly, resulting in the safety valves remaining on for longer periods than might be desired.
Finding the balance between the two can be difficult, as it is in real life!
The other part is how you manage your fire though. When you expect to be working really hard, you'll want the coal level around 100% (this is the optimum coal level, where the fire will be hottest). Where you expect to be working hard soon, but not straight away, you might increase it over 100% (so effectively bank the fire, allowing it to burn down to 100% as you get to where you need it hotter). Where you expect to be working less, you'll keep the coal level lower (resulting in a cooler fire, and hence less steam being generated).
Remember that there is a lag between the fire getting hotter or colder, and the boiler pressure changing. There is a known issue that causes the boiler pressure to not drop when the loco is cold, but not using steam (ie when sitting in a yard), however the safety valves will still use steam up if they do lift.
But this lag means that you need to plan ahead, generally several minutes ahead (depending on the loco, some might need you to plan ahead further). On real locos, how far ahead you need to plan depends on the loco, the route, and the type of coal you are burning. Some coals may need you to plan 10-15 minutes ahead, others might only need you to plan a few minutes ahead. This has, in previously releases, been quite difficult since it was difficult to know the lay of the track ahead (and most of us don't have time to thoroughly learn a route like the real crews would do!). With TRS19, the new track profile display allows you to see the grades ahead of you, allowing you to plan ahead for when you need to add coal, or when to not add coal!
You will of course need to get to know the loco you are driving to work out how long it'll take for the fire to heat up, and then the boiler to heat up.
It was also mentioned about driving practice above; the method you use to drive the loco (ie how to use the regulator and cut-off) can vary a bit between locos. Certainly, when you are wanting to 'power'/accelerate superheated locos work best when you have the regulator as wide open as possible and then drive off the cut-off. Of course when you don't need to power you may still need to adjust the regulator to help to control the train's speed. However saturated (non superheated) locos can vary between wanting you to drive off the cut-off or drive off a combination of the two; opening the regulator too far can drain the boiler far faster than you may want even with a short cut-off, or may simply put down more power than the wheels have traction for.
Regards