On your return flight you may either fly back from Canada to Amsterdam or Paris where you may end your trip and not continue by rail to Belgium. If you consider to book your ticket based on this KLM / Air France promotional sale please look for travel dates between end of August and 31st of October 2015. (based on date of your outbound flight to North America). We recommend to use service of Skyscanner to quickly find out which airlines are available from your hometown. Please refer to below booking samples if interested in this deal to Canada.
Booking samples - look for return flights from Antwerp (ZWE) or Brussels (ZYR)
Belgium - Montreal from €369 (travel sample 26.10. - 6.11. ex Brussels)
Belgium - Montreal from €369 (+€22 booking fee)
Belgium - Montreal from €363
Belgium - Montreal from £260
Travel date samples - The easiest way is to search directly via KLM web site. They offer +- 3 days view so you can easily select travel dates that suit you the most.
Booking sample of cheap return flights from Belgium to Canada (Ottawa) from €363 here. In this case you would be departing from Brussels to Ottawa on 19th of September. You would be then returning back from North America to Europe on 26th of September 2015.