Daramouthe did you ever find a solution? I've run into this same problem and I have a little more info. In this thread Epik was focused on the conveyor but I don't think that's related. In my setup I have the conveyor working fine loading materials onto the freight platform, but I found that as soon as I build/place a locomotive on the rail, the conveyor stops. So it's like the presence of the locomotive causes the freight platform to lose power.
I came across this by accident...I had deleted the locomotive and freight car for some reason before building the station & platform and so I noticed at some point that my freight platform had some material in it before it stopped.
I've tested this scenario by deleting and rebuilding rails, train station, platform, and conveyors. Seems to be the rail cars being the trigger to this problem.