Tongs would work, but I suspect a choker chain would too.
Are there any large trees in the area? If so, you could use the tractor to place the RR ties on one side of the stream, more or less aimed in the right orientation, then setup a fairlead up in a tree on the opposite side of the stream to lift the end of each RR tie and drag it into place across the stream.
I use a logging block/pulley and short tree strap to setup fairleads up trees, and a 5/8" or 3/4" logging rope to do the pulling (with chain/choker on the business end). I move logs, but railroad ties would be a good application too, and easier. The nice thing about this is that you can setup the pulley so that you pull in any direction. Sometimes I pull 90 degrees to the motion of the log, sometimes I pull opposite of the direction of the log. Conceivably you could pull from the same exact side of the stream that the RR ties are delivered to.
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