Actually I think it might be an underpass

Very difficult to tell from the aerial imagery. I think you're right that there's an underpass but I'm not sure that NCN 55 necessarily follows it (the one photo I could find looked very muddy). I've done a very cursory fix but it really needs someone to go and look at it then update OSM accordingly!

I found some photos: the canal is on an aquaduct over the river, so I think there is a bridge over the river, then you walk under canal, then it looks like there is a ramp up to canal (not very clear in photos, more like a hint,  OSM maps show it). I may pass it in June :) .

Regards

NB NCN 55 is an underpass at its red rock lane end but is a rubbish rocky forest path: the canal tow-path is actually better, and also at the adlington end there is no easy way to rejoin canal on its (high) aquaduct as were just steep stone stairs IRC.

so choose road or canal but not NCN55...