Editing EMD FP7 (section)

== Design ==
While EMD's [[EMD E-unit|E-units]] were successful passenger engines, their A1A-A1A wheel arrangement made them less useful in mountainous terrain.{{Citation needed|date=April 2009}} Several railroads had tried EMD's [[EMD F3|F3]] in passenger service, but there was insufficient water capacity in an A-unit fitted with [[dynamic brake]]s. The [[Atchison, Topeka and Santa Fe Railway]]'s solution was to replace the steam generators in A-units with a water tank, and so only fitted steam generators into the B-units. The [[Northern Pacific Railway]]'s solution was to fit extra water tanks into the first [[baggage car]], and to pipe the water to the engines. EMD's solution to the problem was to add the stretched FP7 to its catalog increasing the water storage capacity.

[[File:Amtrak San Francisco Zephyr.jpg|thumb|left|AMTK No. 113 leading two [[EMD SDP40F]]s with the ''[[San Francisco Zephyr]]'' at Yuba Gap, 1975]]

A total of 378 cab-equipped lead [[A unit]]s were built; unlike the freight series, no cabless booster [[B unit]]s were sold. Regular F7B units were sometimes used with FP7 A units, since they, lacking cabs, had more room for water and steam generators. The FP7 and its successor, the [[EMD FP9|FP9]], were offshoots of GM-EMD's highly successful [[EMD F-unit|F-unit]] series of [[cab unit]] freight diesel locomotives.

[[EMD F3|F3s]], [[EMD F7|F7s]], and [[EMD F9|F9s]] equipped for passenger service are not FP-series locomotives, which, although similar in appearance, have distinctive differences. This includes, but not limited to, the greater body length. The extra {{convert|4|ft|m|abbr=on}} of length was added behind the first body-side porthole, and can be recognised by the greater distance between that porthole and the first small carbody filter grille. The corresponding space beneath the body, behind the front [[Bogie|truck]], was also opened up; this either remained an empty space or was filled with a distinctive water tank shaped like a barrel mounted transversely.<ref>{{cite book |last1=Foster |first1=Gerald L. |title=A field guide to trains of North America |date=1996 |publisher=Houghton Mifflin |location=Boston |isbn=0-395-70112-0 |page=98}}</ref>

Over their production run, there were numerous detail changes including the style of side grills (notably on Canadian-built units), carbody louvres, and dynamic brake fan sizing ({{cvt|36|in|m|disp=sqbr}} initially, {{cvt|48|in|m|disp=sqbr}} on later production units). Some railroads such as Southern Pacific and Canadian Pacific outfitted their units with rooftop-mounted icicle breakers for protecting [[dome car]] windows in mountain territory where icicles formed around the roofs of tunnels.