Bulldoze the bus station.In the building menu, go to Roads -> Stations.
You’ll see the usual – bus stop, bus station, tram station.
In the top row of that menu, you can switch from passenger (the icon with the little people) to cargo (the house thingy). If you choose the cargo type, you can build cargo stations.
Take a look in the video here:
http://www.train-fever.com/forums/topic/howto-setup-a-cargo-line/#post-6957
around 00:39.