On July 2, David Bell noticed that tub-with-tails are larger than needed to form switch-engine lanes (see the bobsled-run posting on 24 June 2005). Several smaller still lifes with tub-shaped protrusions can provide the same catalysis; boats, barges, long boats, long barges, etc. can all be used. At right is a revised switch-engine 'bobsled run' using boats as catalysts.
Long barges can be used as a common boundary between two adjacent lanes without any possible interference. Barges are sufficient if traffic in adjacent lanes is in opposite directions, or if the timing of traffic in two parallel lanes can be controlled to avoid mirror-image switch-engine phases. At right is a p3450 'swimmer' -- a switch engine doing laps in a lane made of boats:
