Borealis Infrastructure and Ontario Teachers' Pension Plan are exploring strategic options, they confirmed today, after "a number of investment inquiries".
They said in a statement that a financial adviser will be appointed in due course, though also noted that the review may indeed conclude with no action at all.
In 2010, the Canadian firms jointly acquired the 30-year concession from the transport secretary at the time Philip Hammond, for £2.1bn, to operate and manage the HS1 network. It comprises the 109km high-speed rail line joining London and the Channel Tunnel, on which both the Eurostar and high-speed Southern services operate, as well as four international stations along the route: St Pancras, Stratford, Ebbsfleet and Ashford.
It's the only high-speed railway in the UK at present, with parliament set to ratify plans for the first phase of HS2 early next year.
Borealis and Teachers also own stakes in London City airport, as part of the consortium that bid in the £2bn takeover in February.
The airport was courted by up to five different groups of bidders since it was put on the market by Global Infrastructure Partners (GIP) in August last year.