Well!
While I am not an optician I work with people who have vision issues of various types and I conduct functional vision assessments as part of my job – not to prescribe glasses of course, but to see what issues can be accommodated by other methods. I have been wearing varifocals for over 20 years and would recommend them to anyone. The focus at the bottom is at about 14" and very suitable for reading – but that means that that may be where you have to hold your head relative to a machine dial to read it. The advantage is of course that unlike reading glasses you can usually find an accommodation point for things 20", 30" or any other distance away from your eyes.
The problem is looking down as when you look towards your feet you are looking through the reading part of your glasses and so if you want to focus on your feet for example then you may not find it so easy. The day after I got my first pair we went as a family to Warwick Castle. 'Shall we go up and walk the battlements' sure, and going up was OK. Coming down the spiral staircase was a nightmare! Even now it is faith that gets me down unfamiliar staircases!
With regard to long distances when you look through a pair of single focus distance glasses you may have to adjust your vision to focus while with varifocals you can just move your eye slightly to a different power part of the lens for them to remain effective.
If I change my prescription I am almost immediately at home as I have developed the skill in moving my eye to the right part of the lens to match the power to the task – I don't know if this is a recommended way of going about things, but it works for me.
One issue – driving a loco on a ground level track. Unless it is a large loco you need to look down through the reading part of the lens to see the gauges and if that is not at 14" or thereabouts it will not be in focus so you may need to twist your head and bend over to see clearly.