The major concern is the very steep grade. If you can work out how to get it down to 1:50, it becomes a feasible proposition.
If the space you have available mandates a 1:20 grade between levels, then you could treat it as an operational feature, rather than a problem.
Battery powered locos aren't particularly affected by steep grades, so one with suitably low gearing and sufficient weight could be stationed on the incline in an operation where the train loco is assisted up and down the incline by a battery loco acting as a banker. Maybe with a couple of momentum wagons disguised as brake vans added to the train as well to help control the descent.
If you dislike battery powered steam locos, then a couple of heavy battery powered diesels with suitable low gearing could be used to control descents and assist climbing the grade as well.
While geared steam locos could also be used as bankers on the steeply graded section to keep things under control, keeping more than one loco in steam when running on your own is not really a practical proposition.
Regards,
Graeme