What is the minimum space needed for dual tracks?

Question

The space between the tracks and the outside margin plus total width?

Answer 1

This is a variable.

Different tolerances depending on whether main line or yard track, straight track or curvature and track speed.

Freight cars with open loads in excess of 11 feet 6 inches are considered "wide loads."  When your train has one, it is pointed out in your paperwork.  It requires that the train notify yardmasters of yards that the train has a wide load, because clearances are closer in yard tracks.

Cars with wide loads of unusually wide shipments are not allowed to meet other trains with wide loads in certain areas, because the main line tolerances are too thin in these areas for the trains to meet.  Often times in these double track areas, "wait" orders are issued by the train dispatcher to the trains instructing them not to pass a certain point before a certain time.  Call it an instant schedule.  This will ensure the two trains do not meet in these areas.

But, anything less than 15 feet, center line to center line, is getting pretty tight.  In many older yards, there is just enough room with a very small cushion for an employee to fit between the cars.  It is very claustrophobic, however, especially if one of the rails has equipment moving on it.  You definitely need to keep your wits about you.