A main-line is the important railway that joins two cities, the one the fast trains use. Smaller tracks that leave it for towns along the way are branch lines.

A main-line主線 is是 that joins連接 two cities兩座城市 the important railway的那條重要鐵路，the one the fast trains use就是快車走的那一條。 that leave it從它分出去、for towns along the way通往沿線城鎮的 smaller tracks較小軌道，are是 branch lines支線。

You will see it written as two words (main line) just as often, and with a hyphen when it describes something: a main-line train, a main-line station. Though actually, Americans more often write main line as two words for the railway, and they also use Main Line as a place name for a string of rich towns near Philadelphia — that is not this sense.

You你 just as often也一樣常 will see會看到 it它 written as two words (main line)寫成兩個字（main line），and而 when it describes something當它用來形容東西時 with a hyphen就帶連字號：a main-line train、a main-line station。 Though actually不過其實，Americans美國人 for the railway講鐵路時 more often更常 write main line as two words把 main line 寫成兩個字，and they also而他們也 use把 Main Line as當作 near Philadelphia費城附近 a string of rich towns一串富裕城鎮 a place name的地名——that那 is not不是 this sense這裡講的意思。

There is also a slang verb mainline, to put a drug straight into the blood; that is a different world, and you do not need it to understand the train.

There is also另外也有 a slang verb mainline一個俚語動詞 mainline，to put a drug把毒品 straight直接 into the blood打進血液裡；that那 is a different world是另一個世界，to understand the train要懂火車這個意思 you你 do not need it不需要它。