Why yes, I do have an outside wood fired boiler. I've burned ties in it before, but I had to cut them in half, a full length one was too long. I can only get four foot pieces of wood in mine, and four foot of tie weighs too much.......
I tried to use old bridge timbers, which were far and few between, because of less chance of ballast (rock) being ground into the wood and damaging the saw. Just cutting them with a chainsaw is a job. I figured the best way to cut them was with one of them gas powered demo saws with a carbide tipped blade, but that was too money intensive and I didn't ever try it.
My boiler modulates the fire. I have it set to make 170 degree water, and at that temp the dampers close and the fire goes down to a smolder- which is bad when burning ties. It really doesn't make any difference on creosote buildup while burning these things, as far as my boiler goes. All wood creates the buildup while smoldering. The smoke from the ties is a little acrid compared to the normal hardwood I burn.
I really only used ties for one season, and during the colder parts they really did good, cause the house was always calling for heat and they didn't get much chance to smolder. Actually, I've got cast iron grates in this thing and used to burn coal, and the smoke from that really was bad while it was smoldering. Coal got hard to find around here, so I just burn wood now.