operators within the State. While speed control is used on many passenger lines in the United States, in most cases it has been adopted voluntarily by the railroads that own the lines. Only three freight railroads, Union Pacific, Florida East Coast and CSX Transportation, have adopted any form of ATC on their own networks. The systems on both FEC and CSX work in conjunction with pulse code cab signals, which in the case of CSX was inherited from the Richmond, Fredericksburg and Potomac railroad on its single main line. Union Pacific's was inherited on portions of the Chicago and Northwestern east–west main line and works in conjunction with an early two aspect cab signaling system designed for use with ATC. On CSX and FEC more restrictive cab signal changes require the engineer to initiate a minimum brake application or face a more severe penalty application that will bring the train to a stop. Neither system requires explicit speed control or adherence to a braking curve. The Union Pacific system requires an immediate brake application that cannot be released until the train's speed has been reduced to (for any train traveling above that speed). Then, the train's speed must be further reduced to no more than within 70 seconds of the initial cab signal drop. Failure to apply the brakes for these speed reductions will result in a penalty application. All three freight ATC systems provide the engineer with a degree of latitude in applying brakes in a safe and proper manner, since improper braking can result in a derailment or a runaway. None of the systems are in effect in difficult or mountainous terrain. See also Anti Collision Device Automatic train stop Automatische treinbeïnvloeding Cab signalling Positive train control Train protection system References Train protection systems Further reading The Railway Technical Website: Automatic Train Control