Shore Line East began operations on May 29, 1990 originally as a temporary service during parallel I-95 construction replacing the historical Clamdigger that is the historical New Haven to New London Commuter Train. The line opened two months before the ADA so the original stations were allowed to be much simpler with just bus shelters and tiny wooden boarding platforms. In 1990 the Northeast Corridor wasn't electrified beyond New Haven so Shore Line East began operations with diesel equipment and uses diesel equipment to this day. The operation plans to use M8s eventually. The use of M8s requires the completion of high-level platforms in Westbrook, figuring out a way to board a high-level only train in New London on Track 6, and electrifying a number of sidings that don't currently have catenary wire since they are unused by Amtrak and weren't part of the original electrification to Boston that was finished in 2000. Trains currently consist of 3 or 4 (including a cab car) ex-Virginia Railway Express Mafersa Coaches (acquired by Shore Line East in 1992) that run in push-pull powered by a 1996 built for Shore Line East GP40-2H Locomotive or ex-Amtrak (leased and now owned) Genesis P40 Locomotive (built in 1993).

The core of Shore Line East is the 32.8-mile line between Old Saybrook and New Haven directly connecting in New Haven to Metro-North's New Haven Line for service farther west to Stamford and New York City. Trains make 15 round-trips per weekday, 9 westbound and 10 eastbound trips per weekend (a train must deadhead). Operational requirements at the intermediate stations on the line (except for Guilford) because of platforms on only the southern track of the two-track Northeast Corridor require trains to only stop at the intermediate stops during the peak direction. This is New Haven-bound during AM hours and New London-bound during PM hours, including weekends. 6 of these trips on weekdays, 7 on weekends are extended east to New London (running a total of 50.6 miles). During weekday rush hours 2 trains (and a reverse peak train during the AM only) run through New Haven (with the crew changing in New Haven from Amtrak to Metro-North employees) to Stamford, one of these trains operates through to and from New London making a trip of 89.8 miles.

Ticketing:
Shore Line East is operated by Amtrak and uses their tickets, although discounted UniRail monthly passes in combination with Metro-North can be purchased. Rail Tickets purchased at the New Haven, Old Saybrook, or New London ticket offices look like regular pre-etickets Unreserved Amtrak tickets except with Shoreline/East as the passenger name since the agents don't ask for ID or your name. On board ticket purchases are the same size as the conductor pads used by regular Amtrak on-board transactions except with the Connecticut Commuter Rail Logo on them. In addition the codes of stations available to be punched includes both those along the Shore Line and those of possible stops along a never operated commuter route from Old Saybrook to Hartford.