[lfowles]

You could lengthen the bridge (theoretically, the pieces might not fit that snug) with any combination of the pieces all the way down to the case of a reverse bridge connected at the bottom.

[JoshTriplett]

Unless they fit very snugly, I don't think anything would hold up the extra pieces in the middle, especially not well enough to support a train.

[dublinben]

The original creator of this type of train track offers just such a support piece.[0] You can also 3D print your own supports![1]

[0] http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/DAUAAOSwGvhT3ny3/$_...

[1] http://www.thingiverse.com/thing:110221

[pavel_lishin]

Oh no, we're not combining Lego bricks with this, that's how we got here in the first place!