Hear Hear on that. Always have access to your tunnels. Access from the side, or top it doesn’t matter. Even a reach in from the bottom along the tracks helps. Build your hill or mountain with a lift off of some scenery or buildings, Use Drawer pulls with flip handles to lift them off. Hide the pulls with rock formations that will lift off. Your lift off should overlap the permanent scenery and if done right no one would ever know. Side entries are good especially at the edge of the table where a door or drop down door can swing open. This makes easy cleaning and maintenance of those hidden spots. Be sure to install cross supports to support this scenery or have them installed on your lift off so the remove with the lift off so you have full access along the track.. Keep track connections to a minimum through the tunnel and ensure the rails connect well if you do. Soldier flex track connections to when you lay the rail that the track connection stays lined with the rail and does not jog in the connections. Getting a wrecked train out is tough without access.
My first was under a mountain where I had a small hole back in the back corner and of course crawl under the layout to get to it. Even when putting in my arm past my elbow it was tough to reach the whole wreck. I always conned my younger brother at the time to go retrieve it. Now I know to make sure I have access all the way. Even just a short distance such as a foot from the tunnel portal can be difficult. You will be glad you did. and done right know one will ever know that scenery opens up. If your tracks are on tables that tracks get out of reach from the edge, Leave your self a way in. As you get older those small holes are tougher and tougher to get to and in. Some folks do duck unders for gates. And those are tough when your guests try them so think about that also as you get older. Newman Atkinson

Believe from the start that your trains will regularly derail in tunnels, so design really good openings for access !