setting track direction

Not sure what I need here but here's the problem. I know I can use track markers to identify which direction a train should travel in on a given track. But what if I need a specified train to travel in the opposite direction on the same track for a short distance without it effecting the normal flow of traffic? How can I achieve this as the track markers wont allow it to do so?
Also do track markers effect a given length or the whole length of track that they applied to? IE does it only effect untill it gets to another track marker or point (switch)? Or for the a set distance?

It has only effect untill it meets a switch.
As far as I know, a train will ignore a direction marker if it has no other way to reach its destination. Keep in mind that if there is a route, but it will take it all around the world, it will go around the world instead of ignoring that one marker and taking the 20 meter connection.

If you like to avoid using direction markers, lay down the track in the direction you prefer the train to us it. If you then give a train a point to drive to and it can take multiple routes of the same length, it will prefer to take tracks layed down in the direction is has to travel.
I hope that makes sense to you. It is kinda anoying to describe.

Yes it does make sense. Thanks