No it's not valid IMHO - the doubleback easement alone isn't enough to validate it. Since origin and destination have a routeing point in common (Wrexham itself), only the shortest route, routes up to 3 miles longer, or journeys to and from the common routeing point are valid (the latter obviously not applicable here since the common routeing point is the destination). Via Shrewsbury is much longer than this so there would need to be a local easement to make this longer journey valid, in addition to the doubleback easement to allow doubling back through Gobowen.
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Just looking into it a bit more, Wrexham wasn't always a routeing point. Gobowen used to be associated with Shrewsbury, Chester and Liverpool. This doubleback easement would have made a lot more sense back then, as Gobowen - Shrewsbury - Wrexham would have been permitted by virtue of the through trains to and from common routeing point rule (with Shrewsbury being a common routeing point of both Gobowen and Wrexham), but the doubleback easement would still have been needed to cover the doubleback through Gobowen.
So I think this easement is redundant, or at least needs to be augmented by additional easements (e.g. a local easement or routeing point easement for Gobowen to Shrewsbury), since Wrexham became a routeing point.