The former head of the Shaanxi provincial work safety body angered internet users by smiling at the scene of the accident which killed 36 people.
They then found pictures of him wearing luxury watches, triggering a probe.
Netizens argued that Mr Yang should not have been able to afford the watches on his salary as a civil servant.
Mr Yang was sacked last year by officials in Shaanxi province for "serious wrongdoing" and then expelled from the Communist Party in February.
Responding to criticism that he grinned at the scene of the 26 August crash, he said: "My heart was heavy when I reached the scene... Junior officials appeared nervous when they were updating me on the situation.
"I was trying to get them to relax a little, so maybe, in an unguarded moment, I got a little too relaxed myself."
He also explained that he "used legal income" to buy a number of watches, saying that the most expensive one he owned was worth 35,000 yuan ($5,550, £3,420).
His trial comes amid growing public anger directed towards officials perceived as privileged and corrupt.
President Xi Jinping has initiated a high-profile anti-corruption campaign that has seen senior figures including the former railways minister and a top economic planning agency official felled.
Internet users are also increasingly pursuing those perceived as having done wrong through online exposes and campaigns.
But in recent weeks there have been signs that this has worried the authorities, with a number of journalists arrested for "rumour-mongering" and a high-profile blogger arrested.
30 August 2013 Last updated at 04:40 GMT