6. There is a special train, with a total of 20 stations along the way (including the starting and ending stations). For safety reasons, it is stipulated that passengers who board at the same station cannot disembark at the same station. To ensure that all boarding passengers have seats (one seat per passenger), the train must have at least ( ～) seats.
(A) 90
(B) 100
(C) 110
(D) 120
6. B.

Consider the number of seats required when the train departs from station $j$.

For a fixed $i(1 \leqslant i \leqslant j)$, among the passengers boarding at station $i$, at most $20-j$ people will still be on the train when it passes through station $j$. This is because the passengers boarding at the same station $i$ will get off at different stations, and there are only $20-j$ stations left.

Noting that $i=1,2, \cdots, j$, the train will need at most $j(20-j)$ seats when departing from station $j$.

When $j$ takes values from $1,2, \cdots, 20$, the values of $j(20-j)$ are $19,36,51,64,75,84,91,96,99,100,99, 96,91,84,75,64,51,36,19$. Therefore, 100 seats are sufficient for the train.

Additionally, when $20-i$ people board at station $i(1 \leqslant i \leqslant 10)$, the passengers boarding at the first 10 stations will each have one person getting off at each subsequent station. Thus, when the train departs from station 10, there will be $10 \times 10=100$ passengers on the train, and the train will need 100 seats.
In summary, the train must have at least 100 seats.