I wanted to post in the Infrastructure against delays article but I'm too late, so I'll post it here:  For the mallard operation you can even use passing loops which are smaller than both trains - there are various illustrations of the exact sequence for low speed trains involving cutting off the ends and reshuffling - but at speed this is impractical and the delay horrific.  The solution is to rely on the Lorentz contraction and travel fast enough that both trains are considerably shorter than the loop.