Totally 3 rakes.
That JRU Pass PM is TPTY. So that can be taken up by GTL. 1 rake. Or can be linked to some DEMU RSA.
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Now 2 more rakes of TPTY - GTL 57475 / 76. Changing this TT is not possible since booking for SL would be done already. Even extension wont work out with 2 rakes with...
more... current TT.
So I am thinking maybe new train.
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Or like you said if DEMU RSA can be worked out and given to JRU Pass. Then TPTY's 3 rakes available and current TT will be retained and extended.