anything i build would destroy something else. trade routes everywhere. can workers join the general population of a city or do they just have to sit there?

Capture some unimproved cities.

They sit asleep on a hill top until they can build lumbermills and then rails. And of course improve those newly captured cities. :D

Give them a well-earned rest or paid holiday from your bulging coffers.

If you haven't already, look for any trees in neutral territory that you can chop, even if it's only for 5 shields. Particularly if the trees are around a new AI city that hasn't popped it's borders yet (better you get the production instead of the AI).

You can build roads on all the terrain in your territory. It'll help you to fight open field battles if you're attacked. Also, sometimes the AI will waste time pillaging these mostly useless roads so it'll buy you a little time to rush some units.

Save some for converting tiles after significant techs or civics are discovered, etc. Biology, State property for instance allow considerable alterations of tiles to optimize hammer output in production cities and allow new tiles to go into cottages in commerce centers.

I have to admit I zap alot of workers after I railroad all my tiles up.

maybe gift them to your allies?

Use them to chop forrests outside your borders, build roads throughout your civ, and roads to potential new city sites or other AI civs to help get troops there quicker in case of war.

once tiles are fully developed, i keep a stack of 4 workers on oil wells and aluminum mines to rebuild them immediately when that stupid spy from who-knows-where sabotages them. sometimes, if i have only 1 or 2 irons, i stack them there also. i tell all my extra workers to automatically improve trade routes. this will also send them scurrying to rebuild pastures, resource mines, and other such stuff when the spies hit those areas. i also noticed that, when my cultural borders expand, they rush in and put rails there. this is good to have when it's time to attack your neighbor.