What size screws do you use to attach tie channels from track to rails?
If you are talking about the trestle sets, numbers 110 and 111, for traditional Lionel tubular track, I believe the answer is #4 x 1/2 inch, sheet metal screws in black. The part number is 110-25.
Per Complete Service Manual for Lionel Trains by K-line, 1982 edition, page 412