Derailing on Midwestern Branch

Hello all. Please bare with me as I haven't been involved with TS since TS2010 back in 2010 and now that I'm divorcing, I'm back. But, I am derailing on The Midwestern Branch Line sessions. Particularly the first one to fill the hoppers cars with grain. I'll get to 3-4 filled and the session will tell me "Derailment" and end. I tried coming in both ways and it end the same way. What am I doing wrong? Thank you