The bus stops, the doors hiss open, and your dog freezes. Lifting them aboard may get you moving, but it may make the next trip harder and put you, your dog, and other passengers in a difficult position. Step aside, let the bus go, and find another safe way to travel if you can. Check the operator's current pet rules before planning a later ride, they vary by place and service.
If this is a sudden change or your dog has trouble climbing other steps, ask your veterinarian about pain or mobility. Do not train on a crowded platform where your dog or another traveler might be put at risk. If public transport is essential and your dog cannot cope, get a tailored plan from a qualified, reward-based behavior professional while arranging another route.
Build the journey in pieces
Start at a quiet bus stop, far enough away that your dog can watch a bus pass without distress. Reward calm observation and leave. Over short sessions, work closer only while your dog remains relaxed and can take food. Later, choose a quiet time to stand near an open doorway, then practice boarding without rushing, if the operator permits and the dog is ready. The ride itself comes after those pieces are easy.
Bring a secure lead, water, and a plan to leave at the next stop if your dog becomes overwhelmed. Do not make the first successful ride a long journey. A bus your dog can watch without fear is already progress.
Sources: Dogs Trust: How to travel on public transport with your dog. VCA Animal Hospitals: Fear of Places in Dogs.