At journey start, you drive a Class 52 light engine out of the yard and climb up to cross a bridge over the other tracks, then go down the other side. As you go down the other side, the physics are most peculiar. At the start of this 1.7% down gradient you close the throttle and prepare to apply brakes as loco gathers speed. Except it doesn't. Instead, it quite rapidly slows down and stops. Then it slowly rolls up the hill a bit before stopping again. If you stop at the top of the gradient and release the brakes, the loco starts to accelerate down, then starts to slow down, then speeds up, then slows down.... It's as if it's not sure which direction gradient is going - all very odd! Edit: Just discovered that same problem occurs in GWE - Diesel Legends - journey 5A62 'Return to Old Oak Common' which also uses Class 52.
A similar problem occurs on the other side of the bridge in GWE - Diesel Legends - journey '5A18 Return to Old Oak Common' with a Class 52. Here, if you coast down the top half of the down gradient, the loco slows to a halt. You have to apply power to keep it moving. Once you pass halfway, it behaves as it should, accelerating due to gravity with no power applied. The OSD gradient shows no difference between these two states.
I was intrigued by this post so I tested the above service myself and I got the Class 52 to stop and roll back up the hill just by shutting the throttle off. I could repeat the phenomenon most of the way down the hill. I then tested the same service with a Class 45 and that just rolled down the hill as normal after a brief period of constant speed coasting at the top of the hill. Possibly just strange behaviour from the Class 52 loco and not a gravitational anomaly in the Old Oak Common area but I can’t tell for sure. Most odd. PS4.