Are your walls on the right layer? The cutting feature uses the layer to identify the entity type so that it doesn't cut the floor instead. The walls have to be on the WALLS layer for the cutting to work.

Also, it is an advantage to turn off snap when working with the cutting symbols, especially if you are trying to hit angled walls or walls not exactly on the snap grid.

1. That's highly variable. The length of the platform is typically related to the length of the longest train stopping there, which is again dependent on the number of passengers in peak periods. But I've also traveled to subway stations that are actually to small to accommodate the entire train, so they usually announce on the PA system that if you want to get off, the X rear doors won't open, so start moving. But going with a length approximately similar to the train length itself is a good starting point, at least based on my own experiences.
2. I'm not an expert on that and don't fancy running the calculations myself, but this wikipedia page has some interesting data and formula: https://en.wikipedia.org/wiki/Minimum_railway_curve_radius
3. There is both a dimpled texture (but perhaps not close enough, and it is black) and some nice black&yellow hazard stripes in the @Bitmaps\Tiles\Storyweaver HighSpace folder

When you hide ALL other layers, you also hide the symbol definition layer, which cause all symbols to be hidden, no matter what layer they are on.

Individual images makes life much easier. It is also very helpful if the images have good (but not too long) descriptive file names, and for larger collections, organizing the images into different folders (based on the content of the images, not Metadata like file size or resolution) also helps a lot.

Software like CC3+ (and our competitors) all use the concept of symbols that can be placed, and doesn't have image editor features that allows part of an image to be copied and placed, which is why such combined symbol sheets works poorly. Users need to use an image editor to first extract each symbol and save to a separate file, and then insert that.

Simplest is probably just to copy over the ones from the existing symbol catalogs. If you make sure to have snap on both when picking the origin point for the copy, as well as when you paste it into the destination, it is easy to place it in the precise correct location.

Nice map, but the background repetition steals a bit from it. I would have tried either changing the scale of the background fill, or replace it with something that isn't so noisy, or maybe fade it out a bit (can be done by placing a polygon on top with one of the solid xx fills)

They are installer cache files. CC3+ doesn't need them at all, but installer operations (uninstall/repair) may complain because it can't find the caches source files needed for the operation.

Also note that the Shaded Polygon command changes the fill style to whatever is currently selected. You can use :CC2KEEP: to set the current settings to match that of your poly before running the command. Or you can use the Shaded Polygon (Angle by Edge) command which doesn't do this, and at the same time figures out the correct angle for you.

That should work fine. Make sure that you include the System subdirectory of the CC3+ data directory, as this is where all the settings for the symbol catalog settings, master filters and drawing tools are stored.

Note that symbol catalog settings can also usually be fixed manually quite quickly unless you have a lot of them with problems. The most common issue is the file name of the actual catalog being removed from the setting, but the name of the setting itself should be enough to identify which catalog it needs, so you can browse to it and then save the catalog.

It's an advanced topic, not covered in the manual.

It's described in the help files accompanying the program, and also in the Tome if you own that.