According to UKsteaminfo, it will be running on the following route:
Outwards:York-Normanton-Wakefield-Castleford-York-Scarborough
Return: Scarborough-York-Normanton-Wakefield-Castleford-York
http://www.westcoastrailways.co.uk/sse/Scarborough_Details.html
So that seems, to avoid running round anywhere, it is going to have to come down the Normanton lines from Church Fenton, through Castleford disused platform, Normanton, Wakefield Kirkgate (nice!), then the chord up to Westgate, main line to Leeds, then right at Whitehall, through Woodlesford round the Methley curve then to Castleford (again) then back up to York.
If so, it seems strange that it is going through Wakefield, mainline to Leeds then not picking up in Leeds where I would have thought there would have been a bigger potential market than the likes of Wakefield and Castleford.
Having said that, the chances of it being diverted are probably fairly high as there are cable thefts round Castleford on an almost daily basis and they are never very quick to fix them - especially between Castleford and Milford Jn which is freight and diversions only.