The straphanger who became trapped between a train and a moving platform was a seasoned subway rider — and his family can’t figure out how he ended up where he did.

“We know absolutely nothing,” David Dion, the distraught older brother of Michael Dion, told The Post yesterday. “[Cops] are going to review the [surveillance] tape.”

Dion, 41, remained in extremely serious condition at Bellevue Hospital after being crushed for 30 minutes between the hydraulic station-platform extender and a No. 4 train in the Union Square station at about 10 p.m. Friday.

His father and brother are planning to join him in Manhattan later this week, after his mother undergoes hip-replacement surgery in Arizona today.

A marketing representative with Univision, Dion moved to New York City about 10 years ago from Fitchburg, Mass.

He is unmarried and has no children.