Railtype labels according to the Standardized Railtype Scheme
http://newgrf-specs.tt-wiki.net/wiki/Standardized_Railtype_Scheme

TRACK GAUGE AND TYPE CLASS
All vehicles run on standard gauge track. The early steamers ran on 
broad gauge, but we're not implementing that.
-> Class: S

SPEED LIMIT CLASS
-> Class: A

AXLE LOAD CLASS
This should be processed in the infosheet, simply by dividing the 
engine/wagon weight by the number of axles, assuming a full load.
The lowest axle load in the set is 4 t, the highest is 34 t. All vehicles
are mapped linearly inbetween.
There will be five different axle load classes.
-> Class: A     0-10 t
-> Class: B     10-20 t
-> Class: C     20-30 t
-> Class: D     30-40 t
-> Class: E     40+ t

ENERGY SOURCE TYPE CLASS
There are unelectrified, 1500 V DC electified, 25 kV AC electrified, 
dual system electrified and 3rd rail electrified vehicles in the set.
-> Class: N     unelectrified
-> Class: D     1500 V DC
-> Class: A     25 kV AC
-> Class: E     dual system 1500 V DC and 25 kV AC electrified
-> Class: 3     third rail electrified

STANDARD LABELS
All unelectrified will fallback to RAIL.
All electrified except third rail will fallback to ELRL.

LABEL TABLE
Below is an overview of all railtype labels, in the form of an NML 
railtypetable. Not all of the labels are actually used.

railtypetable {
    unelectrified_A: [SAAN, RAIL],
    unelectrified_B: [SABN, RAIL],
    unelectrified_C: [SACN, RAIL],
    unelectrified_D: [SADN, RAIL],
    unelectrified_E: [SAEN, RAIL],
    DC_A: [SAAD, SAAE, ELRL],
    DC_B: [SABD, SABE, ELRL],
    DC_C: [SACD, SACE, ELRL],
    DC_D: [SADD, SADE, ELRL],
    DC_E: [SAED, SAEE, ELRL],
    AC_A: [SAAA, SAAE, ELRL],
    AC_B: [SABA, SABE, ELRL],
    AC_C: [SACA, SACE, ELRL],
    AC_D: [SADA, SADE, ELRL],
    AC_E: [SAEA, SAEE, ELRL],
    dual_A: [SAAE, ELRL],
    dual_B: [SABE, ELRL],
    dual_C: [SACE, ELRL],
    dual_D: [SADE, ELRL],
    dual_E: [SAEE, ELRL],
    metro_A: [SAA3, MTRO, "3RDR"],
    metro_B: [SAB3, MTRO, "3RDR"],
    metro_C: [SAC3, MTRO, "3RDR"],
    metro_D: [SAD3, MTRO, "3RDR"],
    metro_E: [SAE3, MTRO, "3RDR"]
}