The DCC controlled coupler is shown on the left. Note normal lip on coupler knuckle (Fig 1-A) has been removed on the DCC controlled coupler. (Fig. 1-B) This allows the uncoupler to actuate even under slight tension. File just the lip of the coupler until there is nothing to catch the knuckle. If you remove too much material you will have problems with random uncoupling. Two links of the actuating chain can just be seen under the coupler return spring.