It was only ties and walkway that burned, and only a hundred feet or so. Congress had approved repair funds right after thr fire, but PC wanted none of it. With the bridge, PC had to send a portion of its New England traffic to E-L in Maybrook (per Fed Gov ruling) in an effort to help keep E-L solvent. No bridge meant PC could run all the way to Chicago on its own rails and make more money.
The bridge had hydrants that were drained in winter. They were inactive at time of fire. It was a windy day, blowing east to west, and fire was just far enough out to be near-impossible to reach. Also it is my understanding watchmen were abolished from the bridge not too long before.
No "proof" that PC burned the bridge, but they certainly had motive and opportunity.