Walthers Proto-N trailing truck woes

[tbdanny]

Hi all,

I run a Walthers Proto-N 2-8-8-2 Y3 on my N scale switching layout.  As part of regular operations, I need to back the locomotive down onto its train prior to leaving the yard, however this locomotive can't go for more than an inch in reverse before the trailing truck jumps off the rails.  It goes fine forwards - just reversing is the problem.  I'm considering putting either a weight or a spring on top of it for correction - which would be the best option?

Thanks in advance,

tbdanny