The Rhondda Tunnel Society was given the cash to carry out an initial "tapping survey" on the 3km (1.8 miles) tunnel.
Experts will tap the inside of the tunnel, which runs from Blaencwm to Blaengwynfi in Neath Port Talbot, with long poles to determine its condition.
The society has welcomed the grant.
The tunnel was closed during cutbacks of the UK railway network in the 1960s.
There are calls for the Welsh Government to take it over to move the project forward.
But a feasibility and structural survey needs to take place first, which could cost up to £140,000.
Campaigners and MEP Jill Evans will travel to Brussels in a few weeks to try to secure European funding, and the society will meet with the Heritage Lottery Fund on 19 January.
Chairman Steve Mackey said: "We are very grateful for the grant. We feel things are moving forward particularly with regard to the ownership issue of the tunnel.
"The Welsh Government are starting to show interest and seem supportive. We are very pleased."
A project to reopen a disused Rhondda railway tunnel for walkers and cyclists has been awarded a £10,000 grant from the Welsh Government.