Targaryen and allies vs. The Rock and The Reach:
In The Field of Fire, Aegon, his sisters, their dragons, and their allies attacked an army of Lannisters and Garderners. By the end of battle, House Garderener was completely destroyed, and The Reach was given to House Tyrell.
In "The Spoils of War," Daenerys, Drogon, and the Dothraki ambush an army of Lannisters (The Rock/Westerlands) and Tarlys (The Reach). The Tarlys help the Lannisters take Highgarden in "The Queen's Justice," and now represent The Reach since all the Tyrells are dead.
So like The Field of Fire, The Loot Train Attack was a battle between of Lannisters and The Reach versus Targaryen allies/dragon(s).
Aegon's much smaller army of allies consisted of only 10,000 men, and included the ironborn and men from the Riverlands who surrendered to him earlier in the conquest. Daenerys has the Dothraki army at her side, having lost most of her Westerosi allies (Dornish, Tyrells, Greyjoys) to Cersei.