Is Method variable in a singleton class is thread safe in ruby?

I have singleton class named TransactionCreateTemplate and this class has a instance method with one parameter def mail_payload(json_params) my question is, if we create local variable inside the mail_payload method whether that local variable is thread safe?

class TransactionCreateTemplate
  include Singleton
  def mail_payload(json_params)
    transaction = Transaction.new(params[:name])
  end
end
class Transaction
  def initialize(name)
    @name = name
  end
end

In the above one, whether transaction local variable inside the mail_payload method is thread safe?, since it is singleton class, only one object is created and that same object is used by different threads, i am confused.