"Experimental Thermal variant on the RIA 40, the T40 has reinforced barrels and modified thermal rounds for increased kill power."
The RIA T40 is a thermal light machine gun introduced in SAS: Zombie Assault 4. According to the official in-game description, it is an experimental variant of the RIA 40.
The RIA T40's damage and pierce allow for above-average mobbing, but in terms of Damage per Second, it is outclassed by other LMGs. The T40 has the advantages of a large clip and fairly high damage over time, but it has the disadvantages of a long reload time and heavy weight. Although any class or build could use this weapon, it is most effective when used by a Heavy with a few points into the Heavy Gear skill.
RIA T40 is simply a slightly re-designed version of RIA 40. In addition, it has some glowing red lines, which is a sign to identify thermal-damage-dealing weapon of Rubicon Industries.
Difference on Mobile
RIA T40's ammo box looks more a magazine than an ammo box. Its red lines are placed on different pattern.
As with the RIA T7, the "T" in "T40" most likely stands for "Thermal".