Railroading is applying fabric to furniture so that the weft or filling runs vertically. Turning it 90 degrees from the direction that it came off the loom. When railroaded, a length of fabric can be used to cover an entire sofa back without prominent seams.
Many upholstery fabrics are designed so that they can be railroaded. This means that if you turn the bolt on end and roll out the fabric from left to right, the pattern would be continuous across the roll. Fabric which is woven this way is particularly useful in sofas. If a fabric can be railroaded, this eliminates seams on the outside back of a large sofa. If a fabric is not woven to be railroaded, the fabric is used off the roll.