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.

# Answer

420
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
There are 4 carriages in a train and each carriage has 25 seats. If each carriage could accommodate 10 more passengers, how many passengers would fill up 3 trains?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
420
There are 4 carriages in a train and each carriage has 25 seats. If each carriage could accommodate 10 more passengers, how many passengers would fill up 3 trains?
train