Dover Harbour Train Ferry EMU problem

[Selkent]

Just finished hand building a Dover train ferry out of wood for my new exhibition layout set around Dover harbour in the 1960's. The white cliffs in the back ground took ages. Should be all finished with replica station and Lord Warden Hotel next week ready for its first model railway exhibition at Sevenoaks on 8th July. Just need to connect all the boards together and add the buildings, all scratch built.  Just trying to work out why my BR Green N scale Backmann EMU, goes pull pelt one way and half speed in reverse, any ideas