The wagons you can load itself has a large defect, when its loaded.
To much frictions downhill and they rolls to easy uphill.
When unloaded the wagon rolls easier than loaded downhill.

Ex northfolk southern coal district
scenario monumental part 1 (start in the baile mine)
100 loaded coal wagons and two engine (i take away the 2 engine in the back of the train)
14000 Us ton and 2 es44 engine (0,64 hp per ton)
The first uphill it is 10 per thousand uphill slope.
The train drive 21 mph uphill with 14000 ton ?????????

Then it is downhill 13 per thousand slope.
The train is rolling and you have brake a few times but it feels like the brake is on all the time.
If it would have been 8-10 per thousand had not rolled at all, but the train had stopped after a while.

Have tested the friction on train freight wagons and find the solution to this behavior.
Have tested this on several cars with the same results.

But i hope Dovetails fix this bug very soon.

It seems the Capacity line dont add ton gravity to the wagon just add more friction i think.
But the wagons you dont can add freight (load) in scenario editorn dont have this problem.

Resistance rail wagon = Rolling Friction Coefficient (0,001 train) * Gravity * Mass + Velocity2 * 0.5 * Drag Coefficient (air resistance)* Air Density

Possible solution.
If you opned the Bin file for the coat wagon (in my case "Coal_Gondola_G113.bin")
in Rw_Tools (first unzip Ap files)

Find this line
<Capacity d:type="sFloat32" d:alt_encoding="0000000000005E40" drecision="string">120</Capacity>
Change 120 to 1 (remember 120) (if you set to 0 the cargo does not show on the wagon)

Find this line
<Mass d:type="sFloat32" d:alt_encoding="0000006066663B40" drecision="string">27.4</Mass>
Change 27,4 to 147.4 (27.4+120)

Check this line
<RollingFrictionCoefficient d:type="sFloat32" d:alt_encoding="000000E04D62503F" drecision="string">0.0005</RollingFrictionCoefficient>
it should be between 0.0002 till 0.0010 railway wheels to rails.

I run the same train but with the new values in bin file.
Upphill now 6 mph 2 engine (before 21 mph) With 3 engine 10 mph
Downhill it rolls very easy maybe 3-4 per thousand,
it is enough for it to roll at the same speed without locomotives helps

Now finally the feeling is right I think. The train goes up heavily and rolled easily in the down slopes.

But this solution is not quite good for now is wagon always loaded even is not loaded in the scenerio editor.
So you will now have two Bin files for each wagon.
A regular (old) that can be used for empty wagons
and the new bin file for loaded wagons. (ex "Coal_Gondola_G113_loaded.bin)

There may be problems getting started old scenarios when you have modified the files in Bin
for trains operating in these.

I should also check this line i think for brake
<MaxForcePercentOfVehicleWeight d:type="sFloat32" d:alt_encoding="0000000000805140" drecision="string">70</MaxForcePercentOfVehicleWeight>
I change this to 30 (I got the problem that the brakes were too good on my train)