My favorite 18xx game for six players is two games of 1846 with three players each.
To be clear, you pay the bank the purchase price, and then the bank pays $40 to the Big 4 treasury and $60 to the Michigan Southern treasury.
This is just being pedantic except in the case where someone buys one of these at less than the $100 or $140 it usually costs (because it's the last card left in the draft and the Priority Deal player passes it to the right.) For example, suppose the person to the right of the PD player buys the Big 4 for only $90 (this can happen in our group.) They pay $90 to the bank, and the bank pays $40 to the treasury. If you thought of it as "paying the debt", you might conclude that the bank got cheated, since it only netted $50 on a $60 debt. But that's not how it works.