[ComradeTaco]

Most large infrastructure projects have a large number of trades for a short amount of time Contractors have people,experience and equipment that would be cost prohibitive to maintain full time but can be relatively inexpensive when hired for just their needed duration.

The kinds of work that you describe fall more under maintenance and basic repair, which most public agencies have full time employees that perform that roles. For instance the NYC subway has signaling workers for a constant stream of signal work, track workers to maintain track and sign workers for endless lifecycle replacement of signs. New construction work, like tunnels and stations is done by contractors.