Translation:But the tunnel is too narrow!
In English, "but" and "however" are synonymous.
Please flag it next time you encounter it, thanks :)
For sake of nitpicking (but out of the scope of this course), "however" would be even more advisable in English as it occurs at the beginning of the sentence.
So do "but" and "and." Depending on the type of writing, finding accepted examples of those creating a relationship between two sentence topics isn't very difficult.