First thoughts

I would suspect that your issues are more from moisture rather than smoke damage. 

If there is some residue from smoke, a wipe down with a dry soft cloth should remove most of the residue. Maybe a gentle blowing with dry air for the hard to reach parts. I would also recommend a complete removal of the grease and oils and start with new.  If the odor of smoke is still present, open them up and air them out. 

If it is moisture then at least you will need to clean your electrical contacts, at worst you'll have to replace anything electrical- DCC chips, motors etc. Time in airing out everything is not your friend.

Steve

I would call a Manufacture

I would call one or all the loco's manufactures and see what they say.

DIY Diagnosis and Rehab

Dear ???

What make/model of locos are we talking? Most plastic-shelled locos are not that hard to disassemble, and diagnosing them only takes a patient and methodical approach.

If you could take some pics of one or 2 of the ones that don't run that you'd really like rehab'd, pretty sure the collective brains trust could nut out what's gone wrong...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

PS Agreed, if the Fire Dept has put out a fire in the home, there will have been much water over/around/thru the locos. In contrast, if the heat from the first was enough to "kill" a loco mech, it was likely more than hot enough to melt the plastic shell into a blob. Point being, if the shell survived the heat, then the mech likely did too...

If Moisture is the issue!

Place the locomotives in bags of rice.  The rice will draw the moisture out of the locomotive.  This is also good for cellphones that take a bath.  My wife was the expert in giving her phone a full, immersion bath about once a month. We would remove the battery and put the phone and battery in a bag of rice and let it dry for about a week.  She had two phones that she alternated.  Don't even ask about the multiple protective actions taken to keep the phones out of the water, she overcame them all.

I second Prof Klyzir.

There are so many designs of locomotives that we really need to know what locomotives you have by manufacturer.  At this point we don't know if you are talking steam, diesel, brass or plastic.