Train-overcrowding cause

I was an employee of British Rail, when passenger trains were locomotive-hauled, so extra carriages could be added during busy times.
The cause of overcrowding, both on Irish Rail and British Rail, is the introduction of three- or four-multiple diesel units built in Japan or Korea. The only way they can be strengthened is by attaching another multiple unit, or turning a relief train, which BR used to do in an emergency. However, management do not want to do that in this era of penny pinching. Diesel ‘rail cars’ are only suitable for short-distance commuter journeys.