i watched the train ____ in the distance on the platform and then retured home.

感官動詞 see, watch, observe, notice, look at, hear, listen to, smell, taste, feel + do表示動作的完整性......+doing 表示動作的進行性....這是固定搭配

I watched the train disappear強調的是"看見火車消失"這個動作的完整性

所以disappear用的是動詞原形

PS: 感官動詞 + adj也是對的...所以如果非用disappeared的話....那disappeared就表"消失的"(adj).....但壹般都說the disappeared train表"消失的火車"