The Marston Vale Line, which has 12 stations between Bedford and Bletchley, has been without trains since December.
Operator, London Northwestern Railway (LNR), said the "major milestone" was achieved after Class 150s No. 150137, 150139 and 150141 from Northern were acquired, to replace Vivarail Class 230 trains which can longer operate.
A rail replacement bus service would continue to operate until November.
The test run was carried out under the control of an instructor driver, in partnership with Network Rail, to "ensure the tracks and signals along the line are ready for the fleet to use," the company said.
Network Rail has been running rail head treatment trains, external "to reduce the likelihood of trains sliding on tracks which have been used much less frequently than usual since December".
"An exact date and timetable will be confirmed closer to the time as we continue to 'pass out' drivers on the Class 150s," LNR said.
The operator has taken over from Vivarail, which employed the maintenance staff who fuelled, checked, serviced and repaired the Class 230 trains.
As a result of administrators being called in, LNR previously said "it was no longer safe to operate our Vivarail Class 230s... and we were left with no option but to suspend the service".
|Vivarail Class 230's were used on the line until Vivarail went into administration.|
The 230's were created by rebuilding 1980's ex London Underground D78 Stock