type of metal-cased bullet which expands on impact, 1897, named for Dum-Dum arsenal in Bengal. British army soldiers made them to use against fanatical charges by tribesmen. Outlawed by international declaration, 1899. The place name is literally "hill, mound, battery," cognate with Persian damdama.
It was to stop these fanatics [Ghazi] — and that firstclass fighting man Fuzzy-Wuzzy, of the Soudan and of Somaliland—that the thing known as the Dum-Dum bullet was invented. No ordinary bullet, unless it hits them in a vital part or breaks a leg, will be sufficient to put the brake on these magnificently brave people. ["For Foreign Service: Hints on Soldiering in the Shiny East," London, 1915]
Pile on the brown man's burden
And if ye rouse his hate,
Meet his old fashioned reasons
With Maxims up-to-date;
With shells and dum dum bullets
A hundred times make plain,
The brown man's loss must ever
Imply the white man's gain.
[Henry Labouchère, from "The Brown Man's Burden," 1899]
"principal line of a railway," 1841, from main (adj.) + line (n.). Meaning "affluent area of residence" is by 1917, originally (with capitals) that west of Philadelphia, from the "main line" of the Pennsylvania Railroad which added local stops to a string of backwater towns west of the city late 19c. that helped turn them into fashionable suburbs.
The Main Line, Philadelphia's most famous suburban district, was deliberately conceived in the 1870's and 1880's by the [Pennsylvania] Railroad, which built high-toned housing developments, ran hotels, more or less forced its executives to plunk their estates out there, and created a whole series of somewhat spurious Welsh towns along the railroad tracks. ... Now everybody assumes these all date from 1682, like the Robertses; but as Chestnut Hill people like to say, "nobody but Welsh peasants lived on the Main Line till the Railroad built it up." [Nathaniel Burt, "The Perennial Philadelphians," 1963]
The original station stops were, in order out from the city, Overbrook, Merion, Narberth, Wynnewood, Ardmore, Haverford, Bryn Mawr, Paoli. The train line for commuters along it is the Paoli Local.