They may have enough engines in operating inventory if they actually keep them all maintained properly. Do we really know whether they are adequately maintaining more than the number absolutely needed, so that if they had a few unexpected failures, or a couple of bad grade crossing accidents, they wouldn't immediately be "on the ropes", as it were? Between insufficient funding and bad management decisions, the (pardon the pun) track record on repair and mantenance isn't that great. After all, that's why the freight RRs demanded they run two engines on trains that objectively don't need two.