Are the red and green circles different things (railway signal and chain rail signal)? Do they appear all the time or when alt is active?

They are colored based on the signal color. Green for open, red for reserved and yellow for watch your step as per usual. Chain signals have an extra circle to tell them apart. They appear when you hold rail-related items in your hand like signals or locomotive.

You gave me an idea with the remark about alt and I decided to make it toggleable with alt mode.

Perhaps there can be a setting how big all the icons are? (I find them rather large)