AT means Artillery Tank and the AT-1 tanks were the Soviet Tier-2 tank destroyers and were created based on the T-26 light tanks. It was developed in Leningrad at the Experimental Design Mechanical Department by the Construction bureau from 1935-1936. These tanks were mainly developed as anti-tank warfare machines. Even though two prototypes of the AT1 were built and went to trial in 1936, the AT-1 never went into mass production at all.
The AT-1 was the first in line of the Soviet tank destroyers. The AT-1 was easy to control and manoeuvre and had very good acceleration and can turn easily and quickly. Equipped with the 57mm Zis-8S Tier 4 gun it can easily destroy Tier 2 and Tier 3 tanks. The only disadvantage of the AT1 is its view range, as you always need a spotter inside the tank to look out for enemies.
In terms of mobility and turning radius, AT-1 was better than many other destroyers including the T18, but when it comes to speed in a straight line, T18 is much faster than the AT-1. Some sort of speed is helpful for destroyer tanks, as the Tier2-3 tanks move very fast and if you want to catch up with them to destroy them, you also need to move at a brisk pace. But close combat should be avoided in the AT-1 tanks.
AT-1 cannot afford to take any hits from the enemy as it has a very thin armor of just 15mm and bouncing from enemy projectiles is not possible with the AT-1. One of the biggest disadvantages of the AT-1 is the view range as it has the shortest view range of just 250 m. Hence, AT-1 is dependent on other tanks or spotters to target the enemies.