Free speech doesn't give unlimited protection to libel.

Ars Technica (TFA) claims that the judge's order ignores previous rulings, yet the ones it cites are not on point. They involve politicians and business executives.

These involve purported libel of private figures acquitted of a crime.

The contest rules on the front page say:

Each entry in Amtrak's "Picture our Train" photo contest must be an 8x10 color photo of an Amtrak train displaying the current Amtrak logo. No digital images will be accepted.

It also specifically reminds participants not to trespass:

Remember, SAFETY FIRST! Amtrak reminds the public and especially those who may photograph a train to stay out of danger. It is very important to stay away from tracks, moving trains, yards, railroad structures (such as bridges, trestles, towers and wires) and the railroad right-of-way. Photographers must not trespass on railroad property or on private property adjacent to the railroad.