A1A-A1A
A wheel arrangement for diesel and electric locomotives with two three-axle bogies / trucks, the two outer axles of each bogie are powered and the centre axle is unpowered.
A1A-A1A: six wheels per side, power on four of them
An A1A-A1A locomotive has two bogies, each carrying three axles in a line. Of these six axles total, four are powered (driven by traction motors) and two are unpowered idlers. The powered axles sit on the outside of each bogie; the centre axle rides freely. This arrangement is written in railway notation where A represents a powered axle and 1 represents an unpowered one.
The unpowered centre axle carries weight but transmits no torque to the rails. This serves a specific purpose: it reduces the adhesion weight (the mass pressing the driven wheels to the rail) compared to a fully powered six-axle bogie, which lowers the tractive force and helps prevent wheel slip on poor rail surfaces. The A1A-A1A is therefore favoured on commuter and secondary-line diesel and electric locomotives where sustained high force is less critical than reliable starting and moderate speed operation.
Practical use and regional variants
The arrangement became standard on many medium-power locomotives, especially in European railways. Electric locomotives of this type operate successfully at speeds up to 140 to 160 km/h. The idler axle also reduces unsprung mass and can improve ride quality by distributing load more evenly across three contact points per bogie rather than concentrating it on two.
Maintenance demands differ from fully powered bogies. The unpowered axle requires bearing lubrication and monitoring but needs no gearbox, traction motor, or drive shaft. This makes A1A bogies cheaper to build and simpler to overhaul than AA bogies. However, the geometry introduces complexity: the three-axle arrangement demands careful suspension design to keep all wheels in firm contact with the rail under load and during curves.
The A1A-A1A configuration fell from favour after the 1980s as adhesion control improved through electronic systems, making fully powered bogies (Bo-Bo on electric stock, for example) more cost-effective and flexible. You will encounter A1A-A1A locomotives primarily in heritage fleets and on certain legacy commuter lines where they remain in service.