You might consider laying as many as will fit edge to edge and on a long table, butted to a stop on one end.? Clamp the stack horizontally.? Lay over and affix a jig to guide a routers with a surfacing bit and rout out the dadoes.? The jig would look like the ones made to surface large slabs, but simpler as the router does not need to cut across the entire surface, just two tracks.? The cut would be best done in two passes for each depth of cut: down one side and back up cutting the other side.? Depending on the router and bit size, a full dept cut might be possible.? With two such workstation and two workers cutting, one from each side on first station, a third or fourth person could unload the newly notched ties and load up with blanks on the second station.
It would not be the most fun job to work on, but I think it is a low tech and quick way to go about getting them all prepared for the rails.
