There is no provision for combining the codes A, B, C, D, E, G, R and T. The combination of "H" and "G" is possible. "HG" would therefore be a narrow gauge locomotive, with a mix of adhesion and rack rail drive. However, the editions of the official list of rolling stock published up to 1939 generally defined "HG" as "Locomotive for adhesion and rack rail drive", and classified the standard gauge RHB steam locomotives as HG 1/2. Since 1966, in respect of railcars and tractors, "he" or "hm" has meant pure rack rail drive, and "eh" or "mh" has meant a mix of adhesion and rack rail drive. Steam powered tank locomotives were always given an "E", and the maximum speed of the locomotive was designated with a lower case letter. Thus, an Ea 3/6 was a tank locomotive with vmax > , three coupled drive axles, and three unpowered axles. With steam locomotives, separate driving mechanisms were displayed. So, for example, a Mallet locomotive was named G 2x2/2 or G 2/3+2/2, and not G 4/4 or G 4/5. Up until 1920, standard gauge electric locomotives were given the letter "F" and a lower case letter for the maximum speed level. The class later designated as Be 5/7 was therefore originally named Fb 5/7, and the first Be 4/6 was still designated Fb 2x2/3 as at the date of its delivery. The designation "R" (for "rapid") was originally intended for locomotives with an axle load of under . These lightweight locomotives exert less stress on the rails when they negotiate curves, and were therefore permitted to do so at higher speeds (e.g. , instead of ). With the introduction of the Re 4/4II, the axle load limit was dropped following extensive testing. Several locomotives were later given approval for higher cornering speeds, to which their type designation had not yet been adapted (e.g. the BLS ABDe 4/8 or the ). With the advent of the ETR 470 Pendolino, the class designation "N" was introduced. Thanks to their tilt technology, these multiple unit trains have an even higher cornering speed compared with designation "R" (e.g. , instead of ).