Commuters in Singapore can now add Mastercard as well as Visa cards to their SimplyGo accounts to enable them to use their credit and debit cards (with contactless functions) to make fare payments on the bus as well as on the train. If your card does not come with a contactless function (e.g. OCBC VOYAGE Card), or even if it does, you should be able to add it into your mobile wallet (e.g. Apple Pay) in order to use it on public transports.
Matthew over at The MileLion has put together a very comprehensive list of cards that you should use in order to earn the maximum number of miles. The KrisFlyer UOB Credit Card gives you an equivalent of 3 miles per dollar on these transactions but do remember that UOB cards typically calculate UNI$ based on S$5 brackets. What this means is that if your daily expenditure on public buses and trains do not exceed S$5, there is a good chance that you will not be getting any miles. Personally, I am using my OCBC VOYAGE Card (for 1.2 VMs per S$1) as I am hoping to take advantage of the S$5 rebate for spending S$30.
Another thing that many people do not realise is that you can actually pay for your train and bus fares using a registered card in your mobile wallet (e.g. Apple Pay). Before you get overly excited (especially if you have used something similar in Beijing, Tokyo, London or New York just to name a few), credit cards cannot be enabled with Express Transit Mode in Singapore at the time of writing. What this means is that you will actually have to authenticate before tapping your mobile phone on the reader. If you are using an iPhone X or later, this means that you will have to complete Face ID authentication before you tap your phone towards the reader - not a big problem but when you are part of a long queue trying to get in (or out) of the station in the morning, it can get quite stressful if you do not have it set up properly.