There are also issues with aerodynamics in the long, small (in comparision to those on high speed lines), single bore tunnels, and also with the overhead wires which are higher than on high speed lines to allow the double deck shuttles to pass. The former is actually quite a common problem, and it's not at all uncommon to find lower speed limits through tunnels on conventional lines, particularly with line speeds over 100mph. As has been said, the need to path between Shuttles and Freight trains is also a reason for the low speed.