The rule of buying in advance to save money is not only valid when booking a flight but also for train travel. Train tickets are generally on sale 6 months before the departure date. Some train booking tips from us:
Omio customers traveling this route book their trains from Paris Gare de Lyon to Gare de Lens.
Passengers board the train most frequently from Paris Gare de Lyon, which is located around 1.3 miles (2.1 km) away from the city center, and they get off the train at Gare de Lens, located 0.4 miles (692 m) away from the city center.
Here are some other resources that might have the information you need