Hi highly suspect there is/was a difference between the 2. It may be lost to history. Otherwise why would Lionel bother to re-number
The 346 accessory is uncataloged and was offered to the mail order and department store trade in their economically priced sets. It is known to have been included in one Sears set in 1964 and two Sears sets in 1965.
The 348 was available as a separate sale item from 1966 until the end of the Post War period in 1969.
Maybe the different merchandising paths necessitated the two separate model numbers even though they were identical units.