[trollbridge]

Lumber is transported from sawmills via rail cars to some depot where it will then get trucked to places like Home Depot.

There are some large-scale lumberyards that still have a railroad siding and will get lumber by the carload.

A delivery of lumber for a typical house is a full load (or more than one) via truck, and obviously a train isn't going to come to a construction jobsite. So the llumber at the construction site is already hand picked...

It's no big deal at all to cut off ends to make it fit exactly, but when framing most of the 2x4s, etc. are already the exact size you need. You just trim off the top and bottom plates at the edges of the rooms.