It depends on the context really; I shall take the train tomorrow and I will take the train tomorrow. In these sentences the meaning of both will and shall is the same. Though will would be the most natural choice since shall in this form is percieved as you're speaking older english. If we instead choose to put them in a different context; Will I open a door? and, Shall I open a door? Here will takes on a more retorical functionality as in asking yourself if you are going to open a door or not. Whereas shall in this form is an older version of should and would be correct to use when for example answering the statement; It's hot in here. Another example of that is; Shall we dance? and, Will we dance? Shall in this context is used when asking another person if he/she would like to dance with you. Will on the other hand is in a more dreamy position, like if you're wondering if the other person would accept a dance invite from you. There's other situations that could be explained aswell but I gtg!