I think its useful to consider that increasing adoption as being one of the measures in your change management journey. Concur changes the way people handle their expenses/travel from a habit that they may have been doing for years. If you consider your implementation of Concur as a big change management exercise then you may want to also consider using a framework that helps you get to those high adoption numbers. One would recommend is the Proscii process.
The ADKAR model stands for Awareness, Desire, Knowledge, Ability and Reinforcement. I think right now you have fixed the 'Awareness' about Concur (because your folks understand that this is now policy) but you need to move onto Desire and Knowledge to get those numbers up.
What about sharing short 'Tips and Tricks with Concur' - either in the form of images or short videos? Have brown bag sessions whereby people share how to get the best out of it and get the people that already love Concur in your org to lead them so they can advocate on its behalf.........I have a few more ideas but let me know what you think of these.
Travel leads to expenses and you can always use expense reimbursement speed as a motivator. Enable integration from Travel to Expense and make users link itineraries to rail and air expense types. If they don't, route it for additional approvals. Managers will ask employees to start using Travel to avoid these approvals.
Additionally, you will have a lot more data in Intelligence to identify trends and develop strategies.
Thanks @claudiabelardo! I actually just created an infographic that I think goes along with what you suggest. It's colorful and pithy so I'm hoping the "less is more" will work better than blasting them with 1,000 words they'll never read.