The following riffs and rhythms are useful for playing train imitations, in a similar way to chugging.
Although you can play these in any key, it is preferable to use a low key such as G or A, since these sound most like a train.
Firstly, a train whistle. These should work on any chord near the bottom end of the harp, but he best chords are:
234, 345, and sometimes 123
A <45> warble is also a good whistle, best used at the climax of the song for maximum effect.
Good vibrato is essential to achieve the best results when playing train whistles.
Next, some chugging-style rhythms:
123 123+ (say "nukka" on the draw and "tukka" on the blow)
2 123+ 123+ 1 123+ 123+ (best tongue-blocked)
2 123+ 123+ (good one for playing fast)
2 123 123 2+ 2 123+ 123+ (inspired by Sonny Terry, perhaps the best train songs)
For the above lick, it is important to breath around the harp (by opening your mouth wide), especially on the chords so that you don't run out of breath.
This is by no means an extensive list of possible train licks and rhythms. For a player of advanced beginner or above, creating your own groovy riffs should be relatively easy.
Also, the articulation of each riff can be changed to suit the music. The most important thing is the feel the groove of the music.