ts = peripheral.wrap("back")

--[[
on the tick where the train 
was imminent, but was not present.
and now is present and imminent: 
payload sequence
]]
function ready()
    wasimminent = isimminent
    isimminent = ts.isTrainImminent()
    waspresent = ispresent
    ispresent = ts.isTrainPresent()
    if 
        wasimminent and 
        isimminent and 
        (not waspresent) and 
        ispresent 
    then
        payload()
    end
end
function payload()
    sleep(0.3)
    if ts.isTrainPresent() then
        ts.disassemble()
        sleep(0.3)
        ts.assemble()
    end
end
repeat ready() sleep(0.05) until false