Look up train schedules here: http://www.sbb.ch/
Earliest connections from Mont-Soleil to the Swiss airports:
Departure Mont-Soleil at 6:25
Monday-Friday (Saturday one hour later, Sunday two hours later)
Latest connections from the Swiss Airports to Mont-Soleil:
Arrival Mont-Soleil at 23:44
Friday and Saturday (Sunday-Thursday 3 hours earlier!)
The Décentrale is just a two minutes walk away from the Mont-Soleil station.