From 1960, British Rail procured 16 diesel-mechanical multiple units for branch lines. These 300 hp, four-axle units reached a top speed of 113 km/h and featured a driver's cab at both ends. On some of the lines, primarily located in the west of England, the multiple units were coupled with a driving trailer. However, BR only acquired ten of these. The last of these multiple units, nicknamed "Bubblecars" by railway enthusiasts, was retired after 57 years of service. Thanks to their durable technology and the favorable operating conditions on heritage railways, twelve Class 121 units have been preserved. Different starting processes can be selected with F1: 1x press button = warm start / press button 2x = false start / press button 3x = cold start
F2 slows down the locomotive slowly.
F5 enables the heavy load mode: The diesel notch is always one or two notches up compared with normal operation.
F11 and CV 166 = Value 0, 1, 2 or 3, can switch between different coupler (Coupling Screw link = 0, Buckeye = 1, Dellner/Scharfenberg = 2, BSI = 3).
F14, F18 and CV 170 value 0 or 1, can switch between a "normal announcement" or an announcement "with gong".
17 will bring the locomotive to full stop.
F21 and CV 168 = value 0 or 1, can switch between a "normal" or a "squeaking" wiper.
F28 and CV 164 = Value 0, 1, 2 or 3, can switch between different rail joints
F29 and CV 169 = value 0 or 1, can switch between the "Automatic Warning System (AWS)" or the "Train Protection and Warning System (TPWS)".
CV165 can be used to switch the automatic dropout of the driving gears to idle on/off (switch off = value 0 / switch on = value 1). * The function is enabled by default.
* The duration of the gear before it automatically drops back to idle can be set with CV160 (gear 1 = CV160, gear 2 = CV161, gear 3 = CV162, gear 4 = CV163).