In main window you should use central widget . You have two choices :
Set the button for central widget ( Not so good choice ) :
QPushButton *train_button = new QPushButton(this);
train_button->setText(tr("something"));
setCentralWidget(train_button);
Add a widget and add the button to that widget and set the widget for centralWidget :
QWidget * wdg = new QWidget(this);
QPushButton *train_button = new QPushButton(wdg);
train_button->setText(tr("something"));
setCentralWidget(wdg);
And surely you can use Layouts for your centralWidget:
QWidget * wdg = new QWidget(this);
QVBoxLayout *vlay = new QVBoxLayout(wdg);
QPushButton *btn1 = new QPushButton("btn1");
vlay->addWidget(btn1);
QPushButton *btn2 = new QPushButton("btn2");
vlay->addWidget(btn2);
QPushButton *btn3 = new QPushButton("btn3");
vlay->addWidget(btn3);
wdg->setLayout(vlay);
setCentralWidget(wdg);