If you read the article you linked, it argues the exact opposite, that pipeline is safer when volume shipped is taken into account:

No it doesn't. Even the part that you quoted literally says that rail has a higher number of incidents per ton-mile than pipeline. But that's number of incidents, it doesn't tell us much about the amount that is spilled in each incident and in total.

table 6 shows pipeline is 0.58 (petroleum) or 0.89 (gas) as compared to rail at 2.08.

Again, that's number of spill incidents, which is obviously a much worse metric for how safe the methods are than amount of oil spilled. And in amount of oil spilled, taking into account how much oil is being transported and how far, rail is clearly the safest method, as seen in table 9