The following phases were based on existing sources of EMD SD40-2 phases as well as from what I have observed in exterior detail differences of the GP40-2.

See EMD GP38-GP40 phases for spotting differences between Dash-2 units and earlier units.
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- Phases -

Phase 1 - 1972.04 - 1975.05
-Battery box in front of cab: Bolted, small louvers
-Battery box under cab: Hinged
-ECAFB: Separated from cab
-Steps: Version 1
-Radiator intakes: Square wire grid
-Short hood: 81" length
-Cab side panel: Bolted, 11 bolts along top/bottom
-Pilot/ends: Straight upper end plate corners, no raised coupler cut lever, no front anticlimber
-Rear upper pilot/drop step area: Version 1, inward-facing handrail stanchion supports
-Radiator fans: Flat top
-Exhaust: Raised stack
-Blower duct: Flat, wide, two ribs, no raised walkway
-Frame side sill: Narrow over air reservoirs, wide at ends

Phase 2a - 1976.01 - 1978.12
-Battery box in front of cab: Bolted, large louvers
-Battery box under cab: Lift-off tabs
-ECAFB: Flush with cab
-Steps: Version 2
-Radiator intakes: Corrugated
-Short hood: 88" length, vent on right side
-Cab side panel: Bolted, 13 bolts along top/bottom
-Pilot/ends: Notched upper end plate corners, raised coupler cut lever, front anticlimber (2 versions, depending on railroad)
-Rear upper pilot/drop step area: Version 2, outward-facing handrail stanchion supports

Phase 2b - 1979.01 - 1979.11
-Cab side panel: Welded

Phase 2c - 1979.12 - 1981.03
-Battery box in front of cab: Horizontal rib added (some railroads)
-Radiator fans: Q-type
-Exhaust: Raised silencer, opening at rear

Phase 2d - 1981.05 - 1982.07
-Short hood: Bolted hatch added on top

Phase 3 - 1983.03 - 1986.12
-Battery box in front of cab: Latched, lift-off tabs
-Blower duct: Angled, narrow, no ribs, raised walkway ("laundry chute")
-Exhaust: Raised silencer, opening at front
-Frame side sill: Narrow over entire length

Notes
-ECAFB: Electrical cabinet air filter box (behind cab, left side)
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-Various as-built pilot options: standard footboards (up to 1975), plow (various sizes), sloped (2 types), flat end plate
-Two variations in step spacing, depending on railroad
-Oblong cab vent appears on most units (generally on left side, towards rear) starting with Phase 2a
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All GMD GP40-2(W) units have EMD GP40-2 Phase 1 spotting features.

-Mid- to late-production GP40-2(W) and GP40-2L(W) units have a longer electrical cabinet on the left walkway behind the cab, and a single louvered door at the rear of the hood (replacing a shorter door with bolted panel underneath).
-Late-production GP40-2(W) units have chamfered (rather than rounded) step edges.
-GMD GP40-2(W) units have a standard GP40-2 frame; GP40-2L(W) units are about 3.5" higher due to a taller frame.
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Michael Eby 2009