Water troughs (also known as track pans) were essentially pans of water on the track that locomotives would use to pick up water from without stopping.
http://upload.wikimedia.org/wikipedia/commons/0/0b/LNWR_Ramsbottom_Water_Troughs_1904_edited-2.jpg

In Railcraft, these would serve as an alternative to liquid loaders for steam locomotives. These would save time and fuel, at the cost of materials (and possibly wasting water, since a lot of it would splash out). The water troughs would either serve as a special track or be added onto an existing track. The track must be level and straight. Curves and inclines will cause the trough to not work.

More information, it was discussed and the best implementation was determined to be a block that you place under tracks, that has a portion that sticks up above the track and appears to be filled with water.

This solves the concerns with how you would attach pipes and makes rendering rather trivial.

You can then just put any track you like on top of it.

I rather like this idea.

As discussed on IRC I think this idea has merit as a fun mechanic, but I don't think the time or fuel savings would be that significant. I'm not opposed to it being added though.

I am not opposed, but unlikely to devote much time to the concept.