Each tunnel asset is a little different. Some are built as kits. Others are "fully furnished", but have limitations like grid-snap or overlarge digholes. In case you're unfamiliar, digholes are the objects that turn the terrain invisible so you can see into the tunnel.
An example of a furnished 2 track tunnel:
baldTunnel2Track concrete light-grey <kuid:203201:11026>
Author: FSP
I presume this is going to have a "dighole" included (since it's ancient) and it will snap to the grid. If you don't want this, you can get rid of those features by cloning the asset, renaming it, and removing the snap-grid and dighole tags from the config file.
Or, you can try a different tunnel that doesn't have built-in digholes and grid-snap.
An example of a 1 track tunnel without a built-in dighole and grid-snap.
Indigo Tunnel <kuid:156765:100182>
Author: Simulatortrain
In both of the above mentioned tunnels, you would need to remove your track going through the mountain and replace it with the tunnel track.
However, if you didn't want to do that...
Here are some examples of kits and their authors, along with keywords you can search for:
Keyword: Westwood Tunnel, Westwood Tunnel Bore
Author: Magandy
Keyword: RC Tunnel
Author: Ricomon35
This "kits" are tunnels that have been broken down into their component parts. The portals are in the objects tab, the tunnel in the spline tab. Currently I like this method the best, I feel that it gives me the most freedom and uses the least amount of space. I also don't need to tear up my track, so I can keep the same track shape and type. Lastly, I can install switches inside the portals or tunnel, which can be difficult with tracked tunnels assets. Good stuff.
Finally, if you get frustrated with the digholes, you can use an asset to blackout the tunnel portal. 2 examples listed below for those.
RC Tunnel Shadow Entrance
Rail Access Door Open 10m x 9m
There are many more tunnels out there, so be sure to look around on the DLS and other routes for cool stuff.
And hey, welcome to trainz, Frank.
-Pete