The BB I was a steam locomotive with the Royal Bavarian State Railways (Königlich Bayerische Staatsbahn).

Bavarian BB I
Type and origin Builder Maffei Build date 1896 Total produced 1
Specifications Configuration: ​  • Whyte 0-4-4-0 Gauge 1,435 mm (4 ft 8+1⁄2 in) Driver dia. 1,340 millimetres (4 ft 4+3⁄4 in) Length: ​  • Over beams 16,991 or 17,894 mm (55 ft 9 in or 58 ft 8+1⁄2 in) Axle load 14.2 t (14.0 long tons; 15.7 short tons) Adhesive weight 55.6 t (54.7 long tons; 61.3 short tons) Service weight 55.6 t (54.7 long tons; 61.3 short tons) Water cap. 13.8 or 18.0 m3 (3,000 or 4,000 imp gal; 3,600 or 4,800 US gal) Boiler pressure 14 kgf/cm2 (1.37 MPa; 199 psi) Heating surface: ​  • Firebox 2.07 m2 (22.3 sq ft)  • Evaporative 123.00 m2 (1,324.0 sq ft) Cylinders 4 High-pressure cylinder 415 mm (16+5⁄16 in) Low-pressure cylinder 635 mm (25 in) Piston stroke 630 mm (24+13⁄16 in)
Performance figures Maximum speed 45 or 65 km/h (28 or 40 mph)
Career Numbers Bavaria: 2100 Retired 1924

This one-off loco would have been given the operating number 55 7101 by the Reichsbahn in their initial renumbering plan, but in the final one it did not appear as it had been retired in 1924. It can be seen in section today in the Nuremberg Transport Museum.

The engine was initially equipped with a Bavarian 3 T 13,8 tender, later replaced by a Class 2'2' T 18.