The D7 in "A Train" is really a D13#11 chord, not a flat 5 at all.
This is very common in situations where you have a II7 chord.
The D13#11 harmonically is the 4th mode of the A jazz minor scale.
(Sometimes also called a D lydian b7).
In this situation, try using an E major triad over D7. It works great
as a chord voicing as well as an arpeggio for improvising.
reed
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Reed Kotler
reed@justjazz.com
http://www.justjazz.com