I've just unlocked the "BR185: the freight experience" with less than 50km driven on this loco ... weird
I got the rewards overlay show up at the same time but I think (at least on Xbox), the achievement unlocked at the right distance.
I drove about 50 km in that loco and didn't get the achievement, which is correct since the achievement is for 185 km. I did however get the achievement for 1550 km in the BR155 while walking towards it...