Briancon is the practical railhead for Serre-Chevalier, with trains from the south and night-train links from Paris. Some travellers coming from Paris or Lyon use the Paris-Milan line to Oulx in Italy, then cross to Briancon by bus.
By road, the valley is reached from Grenoble over the Col du Lautaret, from Italy over Montgenevre, or from the south via Gap and the N94. Winter storms can make the high approaches slow, so check pass conditions before committing to a tight transfer.