Yes, you can chant "Nam-myoho-renge-kyo" silently from inside during a bus trip, and it can be considered a part of your daily practice of Nichiren Buddhism. Chanting is a fundamental aspect of Nichiren Buddhism, and it is believed to have various beneficial effects on one's life and spiritual development.
While traditionally, practitioners may chant out loud or with others during group activities, it is also common for individuals to engage in silent chanting during moments when speaking out loud may not be feasible or appropriate. Silent chanting allows you to focus your mind and concentrate on the chant internally, even in public or crowded spaces like a bus.
The key aspect of daily practice in Nichiren Buddhism is the sincerity and depth of your engagement with the mantra, rather than the specific method of chanting. As long as you are genuinely and sincerely reciting "Nam-myoho-renge-kyo" with faith and intent, whether aloud or silently, it can be considered a meaningful part of your practice.
Many practitioners find that incorporating chanting into their daily routines, even during travel or busy times, helps them maintain a sense of connection to their spirituality and inner peace. So, feel free to chant silently during your bus trips or whenever you find a moment for your spiritual practice.