Train timetables are predicated on the need to ensure that train A completes its journey to station X before train B can.
One way to allow express trains and slow trains to travel simultaneously is to provide them with mobile track sections on their roofs (as shown).

This allows a faster train to overtake a slower one -providing extra flexibility and utilisation of a railway network.
Oh damn…just spotted this elegant idea:
http://www.futilitycloset.com/2009/11/18/crashproof/