A road-builder by the sounds of things.
Actually worth reading the report in full:
http://www.nestrans.org.uk/wp-content/uploads/2017/11/FPASTS-1plus-Ellon-Rail-Study_Final-Report.pdf
3 options:
Option 1 - hourly service to Ellon, single track, single platform station at Newmachar
Option 2 - half hourly service to Ellon, single track, dynamic loop and two platform station at Newmachar
Option 3 - half hourly service to P&R east of Ellon, single track, dynamic loop and two platform station at Newmachar, double track from Ellon to P&R.
Options 2&3 also require redoubling and track lowering through the Schoolhill and Hutcheon Street tunnels north of Aberdeen.
Construction costs are shown broadly as £120m, £140m and £170m.
The £381M is taking the Most expensive Option 3 with full Schedule 4, Network rail costs, Track lowering in Aberdeen, Design costs and a 66% Green Book optimism bias.
Even at lower costs though it looks like the BCR is going to be very poor. The three options there generate BCRs in the 0.2-0.3 range so even without Optimism Bias they are well under 1.0.
I still wonder whether a better option might be a Park & Ride site on the outskirts of Oldmeldrum. It was a bonkers seeming scheme suggested a few years ago by Railfuture but would have the definite advantage of 2tph existing terminating services at Inverurie that could be easily extended. At only 6km with 8 major structures and a single platform station I reckon you could bring it in for more like £50-100m.
A large part of the demand at Ellon was for traffic from the north so Oldmeldrum might be able to capture some of that Banffshire demand at a lower cost.