I usually don't automate workers until i'm late into the tech's, and have built most of what I want, ie railroads on mines/lumbermills, and mined all my Aluminuim resources.
Group workers 2 or 4 in a group depending on number of workers and speed of jobs you want.
In the options setting for the game, Check the box "Automated workers DON'T alter/destroy improvements"....VERY IMPORTANT.
I'll usually just automate them over the whole empire when I do....I can't scan the whole continent to see if railroads are all over, etc.
By this stage I've hooked up all resources, built mines, plantations, farms on resources, cottages on areas that I want, workshops with railroads etc. So just set the GROUP of 2/4 etc workers to automation, if nothing to do they'll go sleep in a town.
VERY IMPORTANT, hot key (Control and number key, Control-1, for hot key group 1 or workers) so IF and when the computer AI's pillage, destroy with spies or bomb with a carrrier your improvements, you can send a group or workers there to build the improvement that YOU want/need.
Oh yeah, if playing continents, which I do, you can send a group to your newly conquered lands to build further improvements faster, and start new empire to further your imperial ambitions.