Watch the Midnight Train online guitar lesson by David Blacker from Lone Wolf Blues
In Midnight Train we have a nice steady bass style tune in the key of A. One of the reasons I put this piece together was that unlike some of the other steady bass pieces this incorporates melody chords and notes on all 4 strings D - G - B - E. This helps train the right hand to be prepared for different melody/chordal combinations on the high strings.
In measure 1, there is this great A7 chords with 7th in the bass (5th fret of the D string). This inversion really has an early country blues sound. On the IV chord we leave the A string in the bass to add a little variety to the bass line. The expected bass note would have been a D, but the A being the 5th of D works nicely here. This is meant to demonstrate that you don't always have to keep the root note in the bass - you are free to experiment with what sounds good.
Over the V chord we have a nice chunky E7 shape over the low E in the bass which sounds meaty and strong. The last thing I wanted to point out was the chromatic defending line in measures 3 and 4. Get this sound in your head as it can be used in different octaves and with different chord inversions to create some interesting movement within a piece. Here's an experiment, take a common D7 chord on the top 3 strings and move it up the neck so that the highest notes are on the 9th fret (i.e. 9th free of high E, 8th fret of B and 9th fret of G). Then move this shape down chromatically one fret at a time from 9 to 8 to 7 and resolve it with an A major triad (6th fret of the G, 5th fret of the B, 5th Fret of the high E). Try to find other variations of this other places on the neck.
Recommended listening: Delta Crossroads by Robert Lockwood JR.