<P> Proto scale was originally developed by the Model Railway Study Group in Great Britain in 1966 and later adopted into NMRA standards with modifications necessary for the North American prototype railway standards . Proto scale reproduces faithfully the prototype wheel tread profile and track work used by the Association of American Railroads and the American Railway Engineering Association . </P> <P> Finescale reproduces the prototype wheel tread profile and track work used by the Association of American Railroads and the American Railway Engineering Association with minor compromises for performance and manufacturability . </P> <Table> <Tr> <Th> Scale </Th> <Th> Ratio </Th> <Th> Model gauge </Th> <Th> Notes </Th> </Tr> <Tr> <Td> Z </Td> <Td> 1: 220 </Td> <Td> 6.5 mm (0.256 in) </Td> <Td> NMRA does not give any other dimensions for Z - scale apart from the gauge The s.g. is set nominally to gauge = 6.5 mm; more exact to 1: 220 would be 6.52 mm (0.257 in) </Td> </Tr> <Tr> <Td> Nn2 </Td> <Td> 1: 160 </Td> <Td> 0.177 in (4.5 mm) 4.5 mm (0.177 in) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> Nn3 </Td> <Td> 1: 160 </Td> <Td> 0.256 in (6.5 mm) 6.5 mm (0.256 in) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> </Td> <Td> 1: 160 </Td> <Td> 0.353 in (8.97 mm) </Td> <Td> standard gauge </Td> </Tr> <Tr> <Td> TT </Td> <Td> 1: 120 </Td> <Td> 0.470 in (11.94 mm) 0.472 in (12 mm) 12 mm (0.472 in) </Td> <Td> standard gauge </Td> </Tr> <Tr> <Td> HOn2 or 3.5 mm </Td> <Td> 1: 87.1 </Td> <Td> 7 mm (0.276 in) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> HOn30 or 3.5 mm </Td> <Td> 1.87. 1 </Td> <Td> 9 mm (0.354 in) (0.353 in .) </Td> <Td> narrow gauge--N scale track can be used </Td> </Tr> <Tr> <Td> HOn3 or 3.5 mm </Td> <Td> 1: 87.1 </Td> <Td> 10.5 mm (0.413 in) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> HOm or 3.5 mm </Td> <Td> 1: 87.1 </Td> <Td> 12 mm (0.472 in) </Td> <Td> narrow gauge--TT scale track can be used </Td> </Tr> <Tr> <Td> HO or 3.5 mm </Td> <Td> 1: 87.1 </Td> <Td> 0.65 in (16.5 mm) </Td> <Td> standard gauge </Td> </Tr> <Tr> <Td> OO or 4 mm </Td> <Td> 1: 76.2 </Td> <Td> 0.75 in (19.05 mm) </Td> <Td> standard gauge </Td> </Tr> <Tr> <Td> Sn3 or 3 / 16" </Td> <Td> 1: 64 </Td> <Td> 0.563 in (14.3 mm) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> S or 3 / 16" </Td> <Td> 1: 64 </Td> <Td> 0.883 in (22.43 mm) </Td> <Td> standard gauge </Td> </Tr> <Tr> <Td> On2 or 1 / 4" </Td> <Td> 1: 48 </Td> <Td> 12.7 mm (0.5 in) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> On30 or 1 / 4" </Td> <Td> 1: 48 </Td> <Td> HO - track </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> On3 or 1 / 4" </Td> <Td> 1: 48 </Td> <Td> 0.75 in (19.05 mm) 19.4 mm (0.764 in) (?) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> O or 1 / 4" </Td> <Td> 1: 48 </Td> <Td> 1.25 in (31.75 mm) </Td> <Td> 1.177 in (28.9 mm) is true standard gauge </Td> </Tr> <Tr> <Td> No. 1n3 or 3 / 8" </Td> <Td> 1: 32 </Td> <Td> 1.125 in (28.6 mm) </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> No. 1 or 3 / 8" </Td> <Td> 1: 32 </Td> <Td> 1.766 in (44.85 mm) </Td> <Td> standard gauge </Td> </Tr> <Tr> <Td> Fn3 or 15 mm </Td> <Td> 1: 20.32 </Td> <Td> No. 1 - track </Td> <Td> narrow gauge </Td> </Tr> <Tr> <Td> F or 15 mm </Td> <Td> 1: 20.32 </Td> <Td> 2.781 in (70.69 mm) </Td> <Td> Identical to Proto 20.32 except the wheel flange depth </Td> </Tr> <Tr> <Td> 3 / 4" </Td> <Td> 1: 16 </Td> <Td> 3 ⁄ in (89 mm) </Td> <Td> standard gauge </Td> </Tr> <Tr> <Td> 1" </Td> <Td> 1: 12 </Td> <Td> 4 ⁄ in (121 mm) </Td> <Td>--</Td> </Tr> </Table> <Tr> <Th> Scale </Th> <Th> Ratio </Th> <Th> Model gauge </Th> <Th> Notes </Th> </Tr>

What are the different sizes of model trains