Often yes: on the websites of the railway companies. Or on websites like Rail Europe, Happyrail or Trainline.

Thank you for your reply.  I am a 73-year old that will be visiting my son who is stationed in Brussels.  I am completely unfamiliar with the European train system.  It may be me, but I find it so confusing.  From what I gather, you cannot check on the Eurail website to determine  how much a ticket would actually cost to compare whether it would be worth buying a global pass.  Thus, if I plan  a three-day trip to Paris and a two-day trip to London in January, would it be worth buying a 4-day global pass.  Would checking out either of the sites you listed help determine this?

Thank you for your help, I really do appreciate the time you took to address my concerns.

For just these 2 return trips, I think it's better just to book normal tickets. If you can book early, there are pretty good deals. You can book these on b-europe.com.

Really appreciate the suggestions. Aloha and thank you.