If each carriage had another 10 seats, each carriage would have 25 + 10 = 35 seats.
The train comprises of 4 carriages and each carriage now has 35 seats; therefore a train has 35 x 4 = <<4*35=140>>140 seats.
3 trains with 140 seats each in total can take up 3 x 140 = <<3*140=420>>420 passengers.
#### 420