The Motive Power Industries MPXpress series of North American passenger train locomotives is based on a "platform" concept, in that the basic platform (that is, the shape) stays the same but each component is able to be customized to a specific customer's needs.
Although it it is unmistakably different on the outside, the MPXpress is based on MPI's experience producing both new and remanufactured commuter locomotives to many commuter agencies across North America. It is even available with a host of remanufactured components to help lower costs for any potential customer.
There have been three major variations built so far of the MPXpress: The MP36PH-3S, the MP36PH-3C and the MP40PH-3C. A fourth, the HSP46 is in development for Boston's MBTA.
The breakdown of the name is as follows: Motive Power, 3600 or 4000 horsepower, Passenger, Head-end power equipped, -3 for computerized control systems and Static invertor or Caterpiller or Cummins HEP gen-set equipped.
Other than these, there are many additional options available, such as remanufactured components rather than new ones, different gearings, different brake systems, two different trucks, train number boards, lighting packages, and a wide selection of available HEP packages. With the HSP46, the prime mover and electrical systems will be provided by GE for the first time.
MP36PH-3S
In 2001, Chicago's Metra was looking for a new fleet of locomotives to replace their aging EMD-built F40C's. With GE and EMD as competitors to the bid, MPI won with their design. The first of the 27 MP36PH-3S's that began rolling out of the Boise, ID assembly facility the following year were not MPI's first brand new passenger locomotives however: they had built new F40-variants before, having delivered 12 F40PH-2CAT's for Boston's MBTA in 1995, 5 F40PH-3C's for San Jose's ACE, and 3 F40PH-2C's for Caltrans.
One advantage of a static invertor over an alternator as used before is that the prime mover no longer has to be kept running at the same speed to provide HEP regardless of the output - the invertor can be set to output the power at any frequency necessary. The 3600hp 16V-645FZ3B prime mover can operate as low as Notch 3 and still provide HEP.
Metra's order is to date the only order for the MP36PH-3S, although the option for a static invertor is still available for any agency who may want it.
MP36PH-3C
Shortly after Metra began receiving their order, Caltrain in California put out a tender for 6 new locos to be used on their upcoming "Baby Bullet" service. Once again, MPI won the bid but with a variation: Caltrain's were to have a separate Caterpillar HEP gen-set, rather than the static invertors of Metra's order.
Soon afterwards, additional orders arrived: New Mexico's Rail Runner service ordered 5 MP36PH-3C's. Vancouver's West Coast Express ordered a single unit (equipped with the Canadian-standard twin-bus HEP system). LA's Metrolink and Salt Lake City's FrontRunner put in a combined 22 unit order, with up to 45 additional units available as options to both of those agencies, as well as to Minnesota's Northstar Commuter Rail and Boston's MBTA. Thus far, one option order of 12 units, split three ways between Metrolink, FrontRunner and the Northstar, has been taken. RailRunner's first units, Vancouver's sole unit and much of the first Metrolink order were delivered by 2008. RailRunner's second order of units and the first Northstar locomotive were spotted at the production facility in Boise in August 2008.
Additional orders have since come in as well: Utah's FrontRunner ordered another 10 units for their projected "FrontRunner South" service. Due to the tunnels on the lines around Baltimore and Washington, Virginia Railway Express and MARC ordered modified units that are slightly lower than normal for additional overhead clearances.
MP40PH-3C
In 2006, Toronto's GO Transit put out a tender for new locomotives. The requirements for the new locomotives included being able develop 4000hp, capable of 93mph, and the ability to haul 12-car trains unassisted. Only two bidders responded: MPI and GE.
MPI's offer was the MP40PH-3C. Based on the MPXpress platform it was designed in partnership with EMDI, who supplied the prime mover (a 16V-710G3B-T2), dynamic brake, traction motors and locomotive control system. It also is equipped with a Caterpillar C27-ACERT HEP gen-set.
27 locomotives were delivered by the end of 2008. This was followed by a another 20 units in 2010, 10 more units in 2011, and another 10 in 2014. The last ten locomotives, GOT 657-666, have different exhaust silencers and are Tier-3 compliant.
Sound Transit, the commuter rail operator for the Seattle metropolitan area, purchased 3 locomotives in 2012.
HSP46
In 2009, the MBTA began the process to acquire 20 new locomotives, their first new motive power purchases since 1988. After an extended process that involved re-tendering the contract, MPI was selected on July 14, 2010 to build 20 locomotives of a new type, the HSP46. In 2012 and 2013, the MBTA exercised options for seven and 13 locomotives respectively. The first locomotive was delivered in 2013 and entered service in April 2014.
Much like the MP40PH-3Cs, these units are built from the ground-up with new components. They are the first MPI-built units to use GE prime movers and electrical systems, and also boast AC traction motors. A 12-cylinder GEVO prime mover is configured for 4600hp, and the units use static invertors to supply HEP to the rest of the train. They are geared for 110mph.
MP32PH-3Q
In 2011, the Florida Department of Transportation created SunRail in the Greater Orlando area. Along with ordering several Bombardier BiLevel cars, the FDoT ordered seven locomotives from MPI, designated MP32PH-3Q.
These locomotives are similar to MP36PH-3Cs, however MP32PH-3Q units are refurbished units, having operated as GP40WH locomotives on the MARC network. They are hood unit-style locomotives similar to the GP40WHs, however their cabs have been replaced with the standard MPXpress cab.
MP54AC
The MP54AC is MPI's Tier IV-compliant passenger locomotive offering. It uses two Cummins QSK60 diesel engines that produced a combined 5400 horsepower, but the locomotive can operate on one engine if desired.
MPI offers the MP54AC as new or as a conversion of an older MPXpress locomotive. The first MP54AC produced was a conversion of a MP40PH-3C for GO Transit—delivered in November 2015. It was meant to be a prototype for the conversion of ten more locomotives, however GO Transit has purchased 16 new locomotives instead.
Operators
Canada 
GO Transit - 67 MP40PH-3C
West Coast Express - 1 MP36PH-3C
United States 
Caltrain - 6 MP36PH-3C
FrontRunner - 18 MP36PH-3C
Maryland Area Rail Commuter - 26 MP36PH-3C
Massachusetts Bay Transportation Authority - 2 MP36PH-3C (former FrontRunner units)
Massachusetts Bay Transportation Authority - 40 HSP46 (on order)
Metra - 27 MP36PH-3S
Metrolink - 15 MP36PH-3C
Northstar Commuter Rail - 6 MP36PH-3C-3C (1 former FrontRunner unit)
New Mexico Rail Runner Express - 9 MP36PH-3C
Sound Transit - 3 MP40PH-3C
SunRail - 7 MP32PH-3Q
Virginia Railway Express - 20 MP36PH-3C