Harris between the Dean St. overpass and Atwells Ave. are closest to the tracks. Ugly industrial area with nothing going on, but should be safe during daytime because it has wide-open sightlines. The walk along the river from Providence Place is pretty nice if you want to go there, then cut down a few side streets to Harris. Check on Street View to find the most tree-unobstructed spot, as there are clearings at regular intervals and some are truly excellent vantage points. Harris has 2 freight grade crossings on it: one by Charlotte Hope St. that is for an active P&W customer at a metalworks factory 1 block north on Tingley St. (receives cars during the week, but no idea when). The other is to the Providence Journal Bulletin off Dean St., but they haven't received rolls of newsprint in years.
You'll want to check out the river walk that runs into Providence Place from the east. Not only is it bustling with activity during summer and hands-down one of the best touristy things to do in Providence, but you get this one particularly unique vantage point of the rail tunnel from the Francis St. overpass of the river:
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North of the station, as previously mentioned, the street grid bookends the NEC all the way up to Pawtucket. Your mileage may vary on quality of shots and how far you want to range from Providence Station. Use Street View to survey first so you don't waste too many blocks of walking searching for a tree-unobstructed spot. The P&W freight yard and MBTA layover across the tracks from each other at
Smithfield Ave. are guaranteed to be full of activity, but at 2 miles north of Providence Station that's a bit of a hike if you don't have a rental car or don't have unlimited time to kill for a round trip on RIPTA bus routes 53, 72, or 99 out of Providence Station.