The Le Cœur, Terrance and Lynville Railway was formed in 1878 by the merger of the Chemin de Fer Québec et Trois Rivieres (Québec to Montréal, Montréal to Sherbrooke), the Le Cœur and Terrance Railroad (Québec to Lynville), and the Bangor and Québec Railway (Lynville to Bangor, ME).
The LT&L was always just too big to be driven out of business by rate wars, but too small to be considered a good investment by expansionist railway barons. The era of wildly overbuilding railways passed it by without overextension (plans were made and a route was platted for a route from Ottawa to Toronto but those were shelved during the Panic of 1893, and the overextension and nationalization of the Canadian Northern froze out any interest in building yet another transcontinental.)
By the 1920s, the LT&L had settled into a comfortable & profitable state; a loop of track around the Central Lowland giving two routes between Montréal and Québec, making connections to the Maine Central & Bangor & Aroostook (in Bangor, ME, on the Bangor & Québec), the CNR & CPR at many locations, the NYC southwest of Montréal, the PV&T at the US border (at the southern end of the TdM), the Delaware & Hudson also at the US border, but on the other side of Lake Champlain, and the Central Vermont at Lacolle, PQ.
The LT&L was friendly with all of these railroads, but it was friendliest at all with the PV&T; the two railroads had combined their terminal trackage around Montréal into a common subsidiary – the (as it was called then) Montreal Terminal RR in the 1880s, and did a wide exchange of trackage rights around the turn of the 1900s.
These two railroads gradually entangled themselves in the 20th century; pooling locomotives (at least until the PV&T’s electrification was in full swing), running joint passenger trains, purchasing each other’s stock, and eventually merging into one system in 1961.
Post-merger, the LT&L & TdM initially operated as PV&T subsidiaries, but in the late 1980s (after the D&H acquisition) first the LT&L, then the TdM and D&H, were moved to being direct subsidiaries of the trust that controls the PV&T.
The LT&L is enough of a dominant component of the Parsons Vale system so the corporate HQ has been moved from Boston to Montréal, leaving only a small branch office at the original trust HQ.