Doesn't matter much what it is. If the settings on the wagons are not good, like lots of older freight wagons are, then the problem persists, no matter if you transport fresh air or some families of (un)happy hippos.
Empty car transporters catch quite a lot of wind in real life. So this might be more realistic than you think. The drag causes the train to 'feel' heavier than, for example, a container train which weights twice as much but has better aerodynamics.
That's an interesting thought, actually. Anyhow it provides a good reason to keep the power on and enjoy the lovely sounds of the loco! (Much nicer than that buzzing CD 163 in SimRail!).
Update... Yes it must have been the wagons as I've just started a fresh run on RSN and it is accelerating slightly with power off on the down gradient leaving Hagen. Superb loco!
To add to what others have said already, don't forget that empty trains generally coast much worse than the same trains, but loaded. That's because you have the same drag from air and rolling resistance, but much less mass to counter it with. It's not uncommon for long empty trains to slow down even on slightly descending gradients.
