Kevin Arnott's right boot
Striker
The way that rail franchising works, operators don't own their own rolling stock. As franchises might only last for a few years, it wouldn't be cost effective for a company to buy its own locos and carriages, knowing that they'd be lumbered with them at the end of their franchise, so it's all leased from rolling stock companies (ROSCOs).Here's a thought, why don't they just purchase some more carriages to use at busy times. Therfore meaning that they don't have to turn punters away, which in turn makes more money for them.
Ingenious business idea eh.....
In very simple terms, the DfT and train operator, as part of the franchising agreement, agree what stock will be available throughout the franchise, and what services (as a minimum) need to run. There are all sorts of weird and wonderful anomalies caused by this - an example local to me is that there is an hourly direct service from Coseley (just south of Wolverhampton) to Liverpool, but not a single direct train in the opposite direction. Why? Because the DfT stipulate there should be significantly more services running from Birmingham to Coseley than there are from Coseley to Birmingham, so every Brum-Liverpool service stops at Coseley to meet this franchise commitment, but they don't need to on the way back.
Unfortunately, the DfT aren't going to agree to provide extra carriages that only ever get used for a couple of trains one day per week, and Northern are lumbered with a franchise agreement that they can't change without the DfT agreeing (and no doubt funding).
It will be interesting to see how things change as GBR begins to take over.