The incident happened a day after the Potters Bar crash
The driver of the Banbury-to-Paddington train made an emergency stop, but could not prevent the train hitting the metal posts.
The three-carriage passenger train remained upright - but the posts became wedged underneath the train, damaging an engine oil filter.
Police said children were seen waiting for the impact in nearby bushes and laughing.
A hunt is under way for a group of children, believed to be aged between 10 and 11, seen running across the rail tracks at Hinksey, just outside Oxford.
Police and operator Thames Trains have condemned the "mindless" action.
A British Transport Police spokesman said it was estimated the 1345 BST train was travelling between 40 mph (64.4 km/h) and 50 mph (80.5 km/h).
"The driver had spotted something on the track, but could not stop in time and careered into it.
"Initially, we thought it was a shopping trolley - but it turned out to be what looked like metal five-a-side football posts, about four feet (1.2 metres) by three feet (0.89 metres)."
The posts are believed to have been stolen from a nearby sports field.
A Thames Trains spokesman said: "I find it quite incredible that someone could do something so stupid as to put a metal object like this across a railway line, especially so soon after the accident at Potters Bar.
"I do not even want to speculate what would have happened if it had been going faster.
"I find it very, very upsetting to think that anyone - child, youth or adult - could do something so recklessly silly."
The train was delayed for about 30 minutes and continued to London with one of its three engines shut down.
The train halted before hitting the bar - but Sergeant Sean Boyle of British Transport Police said anyone risking other people's lives in this way could expect "the full weight of the law" to come down on them.