This is a standard classification of railway rolling stock by the UIC (international railway union). Altough it's not adopted worldwide, it is in wide use throughout the EU because it forms a part of the EU Technical Specification for Interoperability (TSI). However, some national deviations still exist.
Although the codes may in some cases look somewhat like real words, they're actually just composed of (groups of) letters, each with a specific meaning. The first (uppercase) letter(s) always denote a general category of the carriage, while the lowercase letters describe its various features (these are not entirely standardized).
See Wikipedia for all the gory details for passengers and
Just a couple of examples of the uppercase letters:
- A: first class passenger coach
- B: second class passenger coach
- E: open high-sided freight wagon
- S: special flat-bed wagon
- Z: tank wagon
So for example, "Bmz" would be a second class coach, longer than 24 meters (m), with a central supply of electricity for sockets (z). Your "Simms" is a flat-bed with a fixed front wall and movable top cover (i), loading length < 15 m (mm), capable of up to 100 km/h (s).