Gnomish, Dwarvish, and Human leaders come together to discuss this for two purposes. The first is in the interest of Allied security. Troop and goods movement is integral in wartime, and it is pertinent to rebuild strong and fast after the last war
The second is national integration into the Alliance and straight up basic diplomacy. Dark Irons see opportunity in the Gnomish desire to fully and finally reinhabit Gnomeregan and request an extension to Shadowforge City. Fearful of the Horde's potential access to such a weak point, in this time of peace, the Alliance (through a network of spies and connections) find that the Blackrock Orcs, recently sated by the recent Kosh'arg and lead by Eitregg, a fellow Blackrock, would stay away from such a project so long as there are joint-rebuilding efforts. These would see Dark Irons, Blackrocks, and Black Drakes working together (not at all in a lovey-dovey hippy way) to rebuild and, indeed, beautify the mountain
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Edit: With the terminus of the Aerie Peak line at Kirthaven coming into such close quarters with the Dragonmaw Orcs, Eitregg pacifies the unrest across the clans by reaching out to Gazlowe and the Bilgewater Cartel in order to contract a similar rail line, connecting Goblin, Troll, Tauren, and Orc centres on Kalimdor.
The unrest cannot be fully quelled, and this leads to conflicts Azeroth's champions can participate in. However, the expansion of the Grimrail is done in a sort of dick-measuring arms race: Between Horde and Alliance, between Gnome and Goblin, between Orc and Troll and Dwarf and Human, et cetera
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With such a success, the Alliance start working on the second phase, bridging the span by connecting Stromgarde through Dun Algaz. The Stormpikes lead the charge, ever suspicious of the Dark Iron fraternization with the Orcs. However, while the Raventusk Trolls are (/wish to be) content in their village, the Vilebranches come to attack the construction happening across the Span. This speeds up the construction process but provides a constant PvP zone in Arathi Highlands (recalling back the olden days)
Reaching Stromgarde, the Wildhammers, Danillian Elves, and the Half Elves conspire with the Gilneans to extend the Tram both west- and north-ward. For the Alliance, this connects the two major continental ports and provides aerial manoeuvrability via the Wildhammers and increasingly sky-bound High Vale Elves
However, Durnholde has never been a stable stronghold, and its station frequently comes under attack by the Alteraci Syndicate (PvP n.2); likewise, Southshore both evolves and devolves into again a stand-off (with great peace potential) with the Forsaken.
This next bit is straight-up headcanon fiction: The extension through Southshore into Gilneas goes through Dun Modr, which gets cleaned up but requires the assistance of Calia Menethil. There, the spirits of Dwarves and Humans are restless (yadda yadda) request to be Lightborne undead. However, the Forsaken feel they have the right to that land (yadda yadda) PvP n.3 in Hillsbrad
