I'd always thought they used the Upper Bristol Road as it is more populated through Newbridge ,as opposed to Lower Bristol Road which goes through very thinly populated bits.The 39/X39 deliberately run down the Upper Bristol Road rather than the Lower Bristol Road so as not to duplicate the train!
Even so , the Windsor Bridge stop is 5/10 mins away from Oldfield Park , although quite why anyone would use the train to Oldfield when the 1/U1/8/12 exist is beyond me