During indicated wheelslip condition, the effective tractive effort is reduced to 25% of it's maximum value. This can be changed on a per session basis using the vehicle physics rule, although I don't see any reason why you would. Within limitations of the current system 25% seems like a reasonable figure.
A few years ago when I was more active with Trainz I did extensive testing on diesel loco physics in Trainz trying to figure out how everything works, how accurate the simulation is and whether or not putting realistic values into an enginespec yielded realistic results. I concluded that for TS12 and TANE the sim could be made quite accurate. For TRS19 I am not so sure.
As you will understand if you read the rest of this post, I concentrated on diesel locos because they are waaaay simpler in enginespec terms than steamers. I figured that until I had a good grasp on accurate diesel loco (and train) physics, there was no point even attempting to wade into steam loco territory.
I have found quite acceptable results using the following parameters for US locos:
surface-area 150
moving-friction-coefficient 0.005
air-drag-coefficient 0.0026
Bear in mind that the surface area should be the frontal cross section of the locomotive. 150 is roughly correct for USA stuff. UK locos are quite a bit smaller and I use a value of 80 for them.
One of the biggest problems with getting faithful physics performance is that there are a wide number of parameters that need to be correct. Not only in the enginespec itself. For example:
In the locomotive config.txt the
mass need to be correct (and in Kg) and the
max-tractive-effort also needs to be correct and in newtons. I see quite a few locomotives (by some very well respected creators too) with the max tractive effort figure written in lbs. In this particular case Trainz need newtons to work correctly!
Also, none of this is any good if our rolling stock enginespecs are all wrong. Unfortunately almost all Trainz rolling stock, regardless of type, region or size, uses the same default wagon enginespec. I have settled for a "best compromise" enginespec for wagons that yields reasonable results in most situations.
In the wagon enginespec I use:
axle-count 4
surface-area 110
moving-friction-coefficient 0.005
air-drag-coefficient 0.0003
The axle count will clearly be wrong for any two axle stock, so they really should have their own enginespec.
Surface area is set at 110 which is a compromise between US and UK loading gauges.
The air-drag-coefficient is set low in order to achieve accurate results through empirical testing. Whole train air resistance is going to get very tricky very fast because the nature of a train means that wagons do not present their full frontal surface area in the direction of travel.
Of course the wagon's mass also need to be set correctly in it's config file (and in the correct units; kilograms). If the wagon is load enabled then this needs to be the empty weight of the wagon.
Finally, any products or commodities to be loaded on the wagon also need the correct value for
mass in their config files. This value should be 1 for bulk commodities such as aggregates or liquids in tank cars, but not for pallets or boxes of goods and I believe should be in kilograms. Again, unfortunately, there are a gazillion different products on the DLS and very many of them have wildly incorrect mass figures which just totally throws any physics accuracy straight out of the window. Quantity before quality apparently.
Once you have all this data set correctly, plugging in the correct data for setting up throttle and dynamic brake curves is the last step and I have created a spreadsheet calculator that can produce these "curves" for a wide variety of locomotives from different regions and manufacturers. Again though, it is only as good as the data you feed it.
So, getting the correct train performance in Trainz depends, not only on the game's physics engine but also the information spread through numerous config files.
Unfortunately, in my experience, whilst we have many talented creators in this community few of them spend the time to get the correct values and units in all of their configs.
Having parameters spread between the enginespec itself and the loco's config.txt is very messy, further exacerbated by the mixing and matching of units of measurement everywhere. Square feet, newtons, Kilonewtons, metres per second. Some consistency would help, but for sure the biggest obstacle is the complete indifference to many content creators in setting up their configs with the correct values in the correct units!