If you only have a 3/4 mile to a train station I would definitely take the bike. At a slow speed you will be sweating way less than walking. At least this is for me. Way less effort to pedal at 5-10mph than walking. No need to race. Can you change clothes at work? Riding in shorts and T-shirt is much better than ring in work clothes. Also can you shower at work? Or are there bathrooms where you can use towels or baby wipes?
I commute in Texas heat and humidity. 5-20 miles one way depending on mood and weather. This morning 94% humidity. In summer I try to start my early morning commute as early as possible. Around 6am. This way I have at least one leg in decent temps. We have the privilege of having showers and I leave my office clothes in a locker and only bring fresh underwear and a fresh shirt.