"I often miss the train."
Translation:Ben sık sık treni kaçırırım.
There is nothing irregular here: to form the Geniş zaman, you have to add a tense suffix AND a personal suffix, not just a personal suffix. The reason of your confuse is that the infinitive for 'yerim' is actually not 'yermek' but 'yemek' ('to eat'). 'Yermek' means 'to disapprove, to criticise', and it becomes 'yererim' in the Geniş zaman.
"Treni sık sık kaçırırım." can be acceptable but yours can't. The differences between "kaçırıyorum" and "kaçırırım" are different tenses, -yor is present continuous tense's suffix and -r, ar, -er, -ır, -ir, -ur, -ür is simple present tense's.
And "kaçmak" is "to escape", "to miss" is "özlemek (means to miss someone, like a friend, a parent, a dear, a pet etc.)" or "kaçırmak (means to miss something/somebody, like kidnapping, a train, a flight etc.)"
I understand your explanation. It's just that in many cases it seems an English sentence that uses the present simple is translated into a Turkish sentence that uses the present continuous. For example, "I like you" = "seni seviyorum".
Since this sentence refers to something that happens often, for me it would make sense to use kaçırıyorum.
Are you sure you haven't? It's the Geniş zaman, the Present Simple tense that was taught in the first lessons. The tense suffix is not -ar (-V²r) but -ır (-V⁴r) because the stem consists of multiple syllables (of two). In comparison, 'yapmak' (means 'to do, to make') has one syllable and turns into 'yaparım'.