My experience was there were quite a lot of ups and down by staircase when we tried to take train to 八景島 via 新杉田 station, there wasn't any elevator, we needed to walk up and down. We had a small boy sitting on stroller and a 5 years old girl, the transfer was totally exhausted. Then when we returned from 八景島 to Shinjuku at about 6pm, the whole train was full of ppl and no seats for us. We were extremely tired. If possible, I would highly recommend travelling by car.
You plan to do both places in 1 day? I think it's not do-able because these two places are located far away. To go to 火車博物館, departing from Shinjuku is way better, while to go to 八景島, 品川 is a bit closer from there.