900 meters.

It being Rome, the Baths of Diocletian are in the way, next to the Museo Nazationale Romano.

Never stayed at the St. Regis, but fabulous frescos in the Museum, so between taking the train, and visiting the museum several times, I've been in the area. Actually, there is more charm there than the average train station neighborhood given the two sights I mention. Beyond that, .mostly a ton of traffic.