You can take train from Hung Hom to Guangzhou. It costs about HKD190 and takes about 2 hours. There is no direct train from HK to Foshan now. You can make transfer at Guangzhou. After arrive at Guangzhou East Railway Station, you can take Guangzhou Metro Line 3(https://www.travelchinaguide.com/cityguides/guangdong/guangzhou/subway/line3.htm) to Tiyu Xi Lu, then transfer to line 1 to Xilang to take Guangfo Line, Guangzhou(https://www.travelchinaguide.com/cityguides/guangdong/guangzhou/subway/guangfo-metro-line.htm) to Foshan.