For anyone thinking of getting smaller magnets than GRJ recommends (1x2x1/2), I tried a 1/2 x 2 x 1/4 that I had around the work bench and this did not quite have the reach for use on traditional track- it is along way from the bottom of the car to the table top under the track, where metal may be lurking.
One thing I did discover: even one of my smaller magnets likes the metal side of the 364 log loader very much! As the car passed the 364, The magnet flew like a rocket out of the gondola car it was resting in for the test I was running to find small nails seeded around the track. What a PITA to remove! 
