The £70m improvement work on the North Cotswold rail line was completed on 22 August.
Parts of the line between Worcester and Oxford have been redoubled with a total of 21 miles of extra track having been laid.
Signalling and stations along the route have also been improved.
The final part of the project was a 16-day blockade in August when 15 miles of double track were aligned, tamped, stressed and welded between Moreton-in-Marsh and Evesham.
The first train ran through the new section at 6am and marks the culmination of a project that started in summer 2009 with double track laid in Chipping Camden Tunnel.