What is a prefix for train?

There isn't a specific prefix that directly means "train" in the same way that "pre" means "before" or "un" means "not". However, depending on what you want to emphasize, you can use prefixes to create words related to trains:

* Rail-: This prefix is common in words related to trains, such as railroad, railcar, railway.

* Ferro-: This prefix is used in words related to iron, which is a key component of trains. You might find it in words like ferromagnetic, ferroconcrete, or ferrocarril (Spanish for railroad).

* Loco-: This prefix relates to locomotives, the engines that power trains. For example, locomotive, locomotor.

Remember, the context is important. The best prefix will depend on what you want to express.