Set in the UK, the story opens with unidentified lumpy parcels on an airport baggage reclaim belt. As I remember it, they become or contain monsters. An armoured train is built to combat the monsters. The train is both solar- and either stellar- or lunar-powered (so it can operate at night). One of the characters cuts a sod of turf from the Manchester United grounds for luck. I can't remember the plot, except an unreliable memory of a bridge having collapsed that they need to repair.
I read this story in the late 90s or early 00s. It was most likely aimed at children, but possibly young adults.
I've searched for various combinations of monster, train, novel, story, lunar power, stellar power, lucky turf, and baggage reclaim to no avail.
I'm not sure it counts as post-apocalyptic. Britain was overrun with monsters but there was enough civilisation to build the train.