The phrase "a main line of" is correct and usable in written English.
It can be used to refer to a primary or significant aspect of a topic, argument, or discussion.
Example: "The report outlines a main line of reasoning that supports the proposed changes in policy."
Alternatives: "a key aspect of" or "a central theme of".
But he allowed that "outlandish as it may be, the book represents a main line of Arab thinking".
The official said a main line of investigation is possible fraud by the funeral home, suggesting operators may have accepted bodies for cremation without actually performing the service.
"The government is going to put a rail system in anyway, so why shouldn't we benefit?" Caliente is on a main line of the Union Pacific Railroad.
But a spokesman for Suffolk police said the two incidents were different, and the possibility that McKeague had been kidnapped "has never been a main line of inquiry".
A main line of questioning ought to be how exactly Mr. King managed to persuade Barclays to get rid of Mr. Diamond.
b5!? in a main line of the Saemisch Variation of the King's Indian Defense was known at least 14 years before Kasparov revived it against Jan Timman in Reykjavik October 1988.
Public buses and trains from MARTA don't help much if you want to get out of the city center or go anywhere off of a main line.
It was planned as the first step of a main line along the South Coast.
A main line track often carries the name of the owning and operating railroad company.
Speaking of reaching out, it has been three years since Ron Kochel and Gene Gulich of Passaic caused a stir at the Abilities Expo in Edison with their adaptive pinball machine, something they had developed almost as an afterthought (their main line of work being a glass-replacement business).
To give an example, suppose a hypothetical company called IntelADP has its headquarters and main plant destroyed by a quake a little off the main line of the San Andreas Fault.
