Design
The locomotive was based on the EMD FP9, lengthened to accommodate additional equipment, including a larger train heating steam boiler. Due to the additional weight of the added equipment and the weight restriction on the Grand Central Terminal Park Avenue Viaduct, the locomotive was equipped with a three-axle rear truck, giving it an uncommon B-A1A wheel arrangement. The middle axle of the rear truck was not powered. The Flexicoil type of truck was used at both front and rear, due to this type of truck having more room for fitting the third rail shoes and associated equipment.
The locomotive was capable of using either an over-running or under-running third rail by means of retractable shoes operated by pneumatic cylinders. The shoes were retracted to the vertical position outside of third rail territory: if extended, electrically live shoes would have projected towards the low-level station platforms that were the norm until the 1970s. A few early examples were fitted with a small DC pantograph for use within New York City's Grand Central Terminal, where long gaps exist in the third rail because of the complex trackage that includes numerous single and double slip switches and double track ladders. These pantographs were soon removed. The complex trackage remains, but the overhead power rails are long gone.
The third rail could be contacted by dropping the third rail shoe and the power source switched at speed, as could be the reverse transition. Unlike some other dual-power locomotives, such as British Rail's Class 73, the diesel engine is the primary source of power. Third rail capability was required only because of the Park Avenue Tunnel and underground Grand Central Terminal, which lack sufficient ventilation for diesel exhaust. The third rail extended from Grand Central Terminal to Woodlawn Junction at the New York City border, where the New Haven diverged from the New York Central Harlem Division. However, dual-power locomotives switch on diesel as soon as possible, immediately after leaving the tunnel, due to diesel fuel being cheaper than electricity for Metro North. The New Haven operated the FL9s from third rail power between Grand Central Terminal and 125th Street Station in upper Manhattan.
Preston Cook has stated that the five-axle configuration was originally designed by EMD to allow additional water tankage on an FP-9 used for long distance passenger trains, and only later adapted the unit to support the additional equipment needed for dual-power operation.
Read more about this topic: EMD FL9
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