I'm not volunteering to help, but I will give you something to think about as I was and still am involved in a small collaborative project.
Your route as you plan it overall should be fairly easy given that much of the European continent has areas of mountains and valleys. The DEM sources would come from SRTM and other European sources. My recommendation is sign up for the services offered by the Geological Survey. They have what is called Earth Explorer.
https://earthexplorer.usgs.gov/
There is an annual review and account re-verification process now, which started a few years ago. There are photos of all areas of the world as well as DEM data for much of it as well. I am not familiar, however, with the European sources so hopefully someone else will wander by and assist you with those.
All of your teammates and including need TransDEM. It's got "that learning curve", but it's worth the investment in the end.
And finally, your double-track mainline is a given for most of Europe and most of that is electrified to boot.
Now on to some project thoughts...
* You will need access to some cloud sharing service such as Microsoft's One Drive, or Google Drive.
This is a must as this data can and will get quite burdensome and your email service is not meant to share large amounts of data.
* Establish a versioning and naming system.
You need to setup right from the beginning a naming convention and methodology so that the route versions are not confused. There's nothing like spending hours fiddling and working on a route, sending it over to the server, then finding out that you've updated a slightly older version. The versioning needs to have dashes and spaces in the file name like this: Enfield and Eastern - ver 2-5 ---- 10-26-2016, or something you establish.
The reason is T:ANE has a bug, that may have also existed previously, which causes "extra" periods from disappearing from the file name when saving a CDP. If you put used something like Enfield and Eastern - ver 2.5 - 10-26.2016, you get Enfield and Eastern - ver 2.cdp. This isn't very helpful when version tracking if all your filenames are the same when writing CDPs as that will overwrite your destination file! (I found this out the hard way and reported it to N3V).
Don't be afraid to use data subfolders. You'll need one for reference photos, shared assets, custom assets, notes, and other data including archives. Yes. As the route grows, you will want to move older version out of the way. I wouldn't delete them until the project is done because there's always that chance that Mr. Murphy will stick a finger in the project and ruin something so having that older part around to go back to at all stages in the project is a good idea.
* Now this brings up another point. Try to keep your CDPs under 350 MB. They can be bigger, but to be on the safe side 350 MB is a good limit. This also helps your teammates too as they download and upload data. Keep in mind not everyone is on Super Blast high speed network (made up the name), and many people in the world have bandwidth limits.
This also has the advantages too since the files will move about quicker and smaller sections can easily be worked on and uploaded again.
* Establish roles.
This might sound mundane, but you need to establish who is going to do which with feedback of course from your team members. Some people might be great at landscaping while others are better at track laying. It's not that others can't pitch in and help, it's just it will help organize things.
* Establish an end goal.
As you've found out route projects tend to drag on forever, heck, we're all guilty of this. I have a personal route which isn't completed and it's been in the process since 2004! It happens, but with a group project you need to establish a beginning and an end.
* Have fun!
When the project becomes a drag, it's no longer fun. Keep up the sense of humor and take the pitfalls in stride.
Now for one final thought. This collaborative project idea fits in perfectly with a service that will be included with N3V's new T:ANE - Next, or whatever the Next is going to become. In some early testing on the Trainz Dev team, we had an opportunity to test a server and build a route. The baseboard area was almost limitless, or seemed that way. Someone laid down some track while others placed textures buildings. I built a small town with a yard, and so the project went. In our case it was more to test the infrastructure, however, it was great not needing to flop files around over the internet as we worked on this project. This service is planned as an adjunct to the main program and offered at a subscription, meaning it can be used to build a project as needed. There is no indication when it will be available so I would use the old-fashion file sharing method for now.