I think it is unlikely the GWR Paddington-Cardiff services will call at Parkway - the turnarounds at Cardiff Central, which include kicking-out time on the down side and a reversal in the Brickyard on the Up side at Cardiff West, are already quite tight, less than 25 minutes in some cases. Given that 15 minutes must be allowed in the turnback, there is nothing to give. Similarly the position in the standard hour of the Up train in particular, either in front of the Cardiff-Nottingham or between it and the Cardiff-Manchester - does not lend itself to insertion of a further 1.5 minute passenger stop at Cardiff Parkway.