Penicuik does have potential and most of the route from Gilmerton to Milton Bridge is reasonably clear.
It never got very close to Penicuik itself but a bus served Park & Ride site would be a reasonable prospect.
The difficulties however are between Gilmerton and Newcraighall.
The new Borders Rail alignment through Shawfair cuts right across the old Penicuik alignment and the power lines south of Shawfair make a new alignment tricky.
Even more problematic is the new road layout through Shawfair and around Sheriffhall Park & Ride which takes no account of any reopening.
Midlothian Council have done a very poor job of safeguarding a future alignment here.
So Penicuik can be done but won't be cheap and will involve expensive changes to recently built infrastructure.
The other route to penicuik was via Bonnyrigg and Rosewell but a small housing scheme on the old goods yard in Bonnyrigg makes that problematic.