Hi,
the first problem may be explained by the fact that it makes no sense to have two lines for the same transportation: the supplier would always choose the fastest line. It would be much better to add trains on the same line and add double tracks (or sidings) with signals to manage the flow.
the second problem is (in my view) one of the most annoying bugs of the game. I think it is caused by the fact that the line is no longer operated (and thus gets over the 20-minutes rule) so that the supplier cancels all transport orders. You can often avoid it by buying a new upgraded train and put it into the line before dismissing the old train. But again this bug sometimes also occurs when you change track layout (breaking the line) even if in pause… I sincerely hope urban games fixes this bug.