Since the turn of the century, mainline U.S. carriers have increasingly deployed the type on transatlantic routes to Europe, and particularly to smaller cities where passenger volumes are insufficient for wide-body aircraft. Production for the 757-200 totaled 913 aircraft, making the type by far the most popular 757 model. At over , , the longest commercial route served by a 757 is United Airlines' Newark to Berlin flight; the aircraft assigned to this route cannot fly with full payload. United's 757s assigned to transatlantic routes are fitted with 169 seats. In July 2018, 611 of the 757-200 versions were in service. The 757-200PF, the production cargo version of the 757-200, entered service with UPS Airlines in 1987. Targeted at the overnight package delivery market, the freighter can carry up to 15 ULD containers or pallets on its main deck, for a volume of up to , while its two lower holds can carry up to of bulk cargo. The maximum revenue payload capability is including container weight. The 757-200PF is specified with a MTOW of for maximal range performance; when fully loaded, the aircraft can fly up to . Because the freighter does not carry any passengers, it can operate transatlantic flights free of ETOPS restrictions. Power is provided by RB211-535E4B engines from Rolls-Royce, or PW2037 and PW2040 engines from Pratt & Whitney. The freighter features a large, upward-opening main deck cargo door on its forward port-side fuselage. Next to this large cargo door is an exit door used by the pilots. All other emergency exits are omitted, and cabin windows and passenger amenities are not available. The main-deck cargo hold has a smooth fiberglass lining, and a fixed rigid barrier with a sliding access door serves as a restraint wall next to the flight deck. Both lower holds can be equipped with a telescoping baggage system to load custom-fitted cargo modules. When equipped for extended-range transatlantic operations, UPS's 757-200PFs feature an upgraded auxiliary power unit, additional cargo bay fire suppression equipment, enhanced avionics, and an optional supplemental fuel tank in the aft lower hold. Total production for the 757-200PF totaled 80 aircraft. The 757-200M, a convertible version capable of carrying cargo and passengers on its main deck, entered service with Royal Nepal Airlines in 1988. Also known as the 757-200 Combi, the type retains the passenger windows and cabin doors of the 757-200, while adding a forward port-side cargo door in the manner of the 757-200PF.