Prior to the Ebbw Vale reopening the Cardiff Curve, and Gear Single weren’t passed for regular passenger use as obviously passenger services ceased in the 60s.
ironically PJ2 signal the one which controls access via the “main“ route Park to Gear had a full length semphore arm, vice PJ9 which only had a 3ft goods arm for the road via Ebbw Junction.
When Newport panel was moved to Cardiff, PJ84 signal up direction off the Gear branch had to moved a significant distance back from its original position to allow the correct modern overlap.
There hasn’t ever been physical reason why regular passenger trains couldn't use both cords from the Mainline to Park Junction (since Newport moved to Cardiff, and Ebbw reopened). The only reason why it hasn’t happened was funding, resources and capacity at Newport.
Since the 12/12/21 the shuttle Cross Keys to Newport arrives and departs Newport plat 4, running via the UP Main via Hillfield Tunnel there and back. This allows a UP train to run in along side from Gear into plat 3 via Hillfield and the DN Main with a Cross Keys or Newport train.
As an interesting side point, IF the Newport West Parkway is we’ve built and has the proposed bay the Gear cord will again fall into disuse as the shuttles will terminate at NWP via Newport High Street.
Park is due to close with its control area passed to the Ebbw workstation at Cardiff in 2023.
I really can't see any benefit in the current shuttle to be honest, it carries more crew than paying passengers, eats up capacity, resources and will be a easy cut when there is a shortage if crew or units or disruption which will lead to passengers not trusting the reliability - its a wholly political service which serves to show “we did something” rather than a real purpose.