AFAIK there is possible to transform two way train line to a 4 way train line, having the high speed trains move on the high speed lines, and the low speed traqins on the low speed lines.
IF and only IF a fast train trails closely a slow train, it will use this bypass to overtake it, if the bypass is long enough. It has to be long enough in order of this to work. A good place to build this kind of bypass is around a station where the slow train stops but the fast one does not.
Note: this is very difficult to pull off.