Sorry to be so right about your Tunnel! But you have come up with an elegant solution albeit with a lot of work and metal. Good job those blocks are loose fit jobbies that make changes much easier.
Noted you talking about Ballast Glue, external PVA let down 50/50 with water and a litte washing up liquid will work just fine. A good spray (water and a little washing up liquid) prior to slopping the glue in place helps it to bed in (use an old Glue Container with a hole 3-5 mm approx to apply being careful around points and none on track surfaces). It is fine to use a little Oil on moving parts to discourage glue sticking them. It will be milky coloured on first appication but will dry clear, also best done on a warm sunny day so that it dries in good time. Overnight drying can sometimes screw up the proces, particularly if there has been any ground frost. Sometime a second application may be required to get it good and solid.
In future if you need to lift your track it will come up easily and a clean using a Steam Wallaper Stripper will get it all off.
Noted you talking about Ballast Glue, external PVA let down 50/50 with water and a litte washing up liquid will work just fine. A good spray (water and a little washing up liquid) prior to slopping the glue in place helps it to bed in (use an old Glue Container with a hole 3-5 mm approx to apply being careful around points and none on track surfaces). It is fine to use a little Oil on moving parts to discourage glue sticking them. It will be milky coloured on first appication but will dry clear, also best done on a warm sunny day so that it dries in good time. Overnight drying can sometimes screw up the proces, particularly if there has been any ground frost. Sometime a second application may be required to get it good and solid.
In future if you need to lift your track it will come up easily and a clean using a Steam Wallaper Stripper will get it all off.